Support Windows bazel builds

[dgoffredo]

Envoy supports Windows, but without Datadog tracing.

If we can get dd-trace-cpp to build and test on Windows via bazel, then Envoy Windows builds could use the Datadog tracer.

See envoyproxy/envoy#31649.

[sujeets-toast]

Hi Any update on this issue?

[dgoffredo]

Hi Any update on this issue?

See envoyproxy/envoy#31649 (comment).

[dmehala]

Done in #119. At least the dd-trace-cpp side.