Improve jacoco coverage in datasources module.

[vamsimanohar]

Is your feature request related to a problem?
In the current state, test coverage is not present for Rest Action handlers in datasources module. It requires use of
testImplementation group: 'org.opensearch.test', name: 'framework', version: "${opensearch_version}". This library is causing issues with existing Mockito Framework and all the other UTs are failing.

What solution would you like?
Incorporate testImplementation group: 'org.opensearch.test', name: 'framework', version: "${opensearch_version}" and fix UTs in datasources module.

What alternatives have you considered?
We can create UTs without relying on testImplementation group: 'org.opensearch.test', name: 'framework', version: "${opensearch_version}"