feat: Add opt-in flag and ClientInterceptor to propagate trace context for Spanner server side tracing

[nareshz]

This PR add changes required for enabling tracing on Spanner server side. The changes include:

• Add x-goog-spanner-end-to-end-tracing header to Spanner RPC contexts.

  The header is added to support server side tracing feature, which aims to create trace spans at Spanner layer to provide more context to customers on their requests to Spanner.

  This feature is off by default and can be enabled by enabling server side tracing while creating Spanner object.

  Spanner spanner = SpannerOptions.newBuilder().enableServerSideTracing().build().getService();

• For Server side tracing feature, traceparent header is required on the Spanner layer in all the requests. This change help in passing traceparent header inside Spanner requests if Context propagators are setup in OpenTelemetrySdk. These traceparent headers will be only added if client has opted-in for server side tracing by setting the enableServerSideTracing option in SpannerOptions.

[olavloite]

LGTM, with a couple of minor nits.

Oh, and please also fix the clirr build failure. For that, you need to add an exception to the clirr-ignored-differences.xml file.