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

[nareshz]

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

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

  The header is added to support spanner 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 any of the following ways:

  1. Opting-in while creating Spanner object.

     Spanner spanner = SpannerOptions.newBuilder().setEnableEndToEndTracing(true).build().getService();

  2. Set the SPANNER_ENABLE_END_TO_END_TRACING environment variable to true.

• For Spanner tracing feature, traceparent header is required on the Spanner layer in all the requests. This change helps in passing traceparent header inside Spanner requests if Context propagators are setup in OpenTelemetrySdk. traceparent header will be only added if client has opted-in for spanner tracing by setting the setEnableEndToEndTracing 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.