Support gRPC Headers from ClientOptions and correct Scope Handling

[natemort]

Allow additional headers to be added via ClientOptions, similar to TChannel. Additionally support removing headers by setting built-in headers values to null or an empty string.

Refactor OpenTracingInterceptor to a separate file and correct Scope handling to resolve Context mismatches. Leaving the Scope open during initialization and then closing it during onClose is incorrect and can cause issues with other interceptors. It's difficult to test this change in isolation.

What changed?

• Support adding headers to gRPC via ClientOptions
• Resolve incorrect Scope handling in open tracing that can cause issues in other gRPC interceptors.

Why?

• Improve gRPC support.

How did you test it?

• Unit/Integration tests and running our validation suite for 24 hours.

Potential risks

• Low. The header functionality adds a missing feature and the Scope changes are low risk. The new structure reduces the likelihood of any conflicts.

Release notes

Documentation Changes