[GO] Add additional AWS v4 signature config options to go client generator

[kpiotrowski]

Golang client generator allows to include the AWS v4 signature. However, It's not possible to provide AWS session token (required when we use temporary credentials) and some configuration parameters like region and service name are hardcoded.

This PR adds additional configuration options: SessionToken, Region and Service to the existing sigv4 implementation.

This change maintain backward compatibility. Old, hardcoded values for region and service name are used as a default ones.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
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[wing328]

LGMT

[wing328]

@kpiotrowski thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423