Modify the gradle plugin to also read from extensions

[ryanbogan]

Currently, the OpenSearch gradle plugin only reads from plugins, but in order to write integration tests for opensearch-sdk-java it needs to also read from extensions.

Action items for this issue:

testclusters folder:

• Create file similar to ExtensionsSettings that can hold extension information

OpenSearchNode:

• Create method to take extensions and write to extensions.yml
• Within this method, add a line in config/jvm.options to enable the extensions feature flag

SDK build.gradle:

• Create integTest task, which contains dependsOn(helloWorld) at the beginning (This will run the extension before integration tests start)
• Set up test clusters