Add tracing for standalone activity client calls

[kritibehl]

What was changed

Implemented ExecuteActivity on tracingClientOutboundInterceptor so standalone activity client calls create a StartActivity span and write tracing context to the header.

Why?

Standalone activity client calls currently bypass the tracing client outbound interceptor because the tracing interceptor does not implement the standalone activity method. This means client-side tracing spans are not created for standalone activity execution.

Checklist

1. Closes Standalone Activities - tracing interceptor doesn't intercept standalone activities client calls #2286

2. How was this tested:

• go test ./interceptor/... -count=1
• go test ./internal -run TestExecuteActivityHeaderAvailableToInterceptors -count=1
• go test ./... -count=1

3. Any docs updates needed?
   No docs updates needed.

[Quinn-With-Two-Ns]

There is already a PR open for this #2302

[kritibehl]

Thanks for pointing that out, I missed #2302.