Reuse sigv4 signing interceptor between opensearch source and sink, include unit tests

[graytaylor0]

Is your feature request related to a problem? Please describe.
This class referenced here (91fabf4#r1213996924) is copy pasted between the opensearch sink and opensearch source for sigv4 signing, and it does not contain any unit tests.

Describe the solution you'd like
Move this class to the aws-plugin-api via an extension point for reuse between the opensearch source and sink, and anywhere else it is needed in the future. Also add unit tests once this class is moved to the aws-plugin-api

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[graytaylor0]

Closing as this class is not being reused in the opensearch source and is not needed anymore with changes to the opensearch java client.