[Refactor] Extract GrpcChannelFactory from GRPCIndexBase

[jhamon]

Problem

I'm preparing to implement asyncio for the data plane, and I had a need to extract some of this grpc channel configuration into a spot where it could be reused more easily across both sync and async implementations.

Solution

• Extract GrpcChannelFactory from GRPCIndexBase
• Add some unit tests for this new class

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: Refactoring only, should be no functional change