[BUG] Cannot install index template

[PSjoe]

Describe the bug
When trying to install an index template using Logstash 7.17 or 8.2 and the opensearch plugin to an AWS Opensearch cluster, we get the following error message:

[2022-05-31T14:30:41,259][INFO ][logstash.outputs.opensearch][main] Using mapping template from {:path=>"/etc/logstash/es-templates/finance-reporting-tool-v1.json"}
[2022-05-31T14:30:41,362][WARN ][logstash.outputs.opensearch][main] Restored connection to OpenSearch instance {:url=>"https://search-purestake-main-cluster-u42s23ct42suzqq4dzjd65biga.us-east-2.es.amazonaws.com:443/"}
[2022-05-31T14:30:41,417][INFO ][logstash.outputs.opensearch][main] Installing OpenSearch template {:name=>"finance-reporting-tool-v1"}
[2022-05-31T14:30:41,433][INFO ][logstash.outputs.opensearch][main] Cluster version determined (7.10.2) {:version=>7}
[2022-05-31T14:30:41,449][ERROR][logstash.outputs.opensearch][main] Failed to install template {:message=>"bad component(expected absolute path component): _template/finance-reporting-tool-v1",

To Reproduce
Steps to reproduce the behavior:

1. Install logstash 8.2.2 or logstash 7.17.3
2. Install plugin logstash-output-opensearch (1.2.0)
3. Configure an output such as:

  opensearch {
      hosts => ["https://redacted.es.amazonaws.com:443"]
      auth_type => {
        type => 'aws_iam'
        aws_access_key_id => 'redacted'
        aws_secret_access_key => 'redacted'
        region => 'us-east-2'
      }
      index => "prod-logs-v1-%{+YYYY.MM}"
      template => "/etc/logstash/es-templates/prod-logs-v1.json"
      template_name => 'prod-logs-v1'
      template_overwrite => true
    }

5. Error will show up when logstash starts.

Expected behavior
Index template should be installed

Plugins
logstash-output-opensearch (1.2.0)

Host/Environment (please complete the following information):

• Logstash docker image
• Version 8.2.2

Full Error Message

Failed to install template {:message=>"bad component(expected absolute path component): _template/prod-k8s-default-v1", :exception=>URI::InvalidComponentError, :backtrace=>["/usr/share/logstash/vendor/jruby/lib/ruby/stdlib/uri/generic.rb:771:in `check_path'", "/usr/share/logstash/vendor/jruby/lib/ruby/stdlib/uri/generic.rb:819:in `path='", "/usr/share/logstash/vendor/jruby/lib/ruby/stdlib/uri/generic.rb:193:in `initialize'", "/usr/share/logstash/vendor/jruby/lib/ruby/stdlib/uri/generic.rb:138:in `build'", "/usr/share/logstash/vendor/jruby/lib/ruby/stdlib/uri/http.rb:62:in `build'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client/manticore_adapter.rb:176:in `sign_aws_request'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client/manticore_adapter.rb:154:in `perform_request'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client/pool.rb:270:in `perform_request_to_url'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client/pool.rb:257:in `block in perform_request'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client/pool.rb:346:in `with_connection'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client/pool.rb:256:in `perform_request'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client/pool.rb:264:in `block in Pool'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client.rb:388:in `template_put'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/http_client.rb:82:in `template_install'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/template_manager.rb:37:in `install'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch/template_manager.rb:25:in `install_template'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch.rb:412:in `install_template'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch.rb:247:in `finish_register'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/outputs/opensearch.rb:224:in `block in register'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-opensearch-1.2.0-java/lib/logstash/plugin_mixins/opensearch/common.rb:83:in `block in after_successful_connection'"]}