Add header matching for gRPC requests to the Java DSL

[fcristovao]

This is a simple PR to add the ability to do Header matching in the Wiremock gRPC calls.
The ability to do the actual matching was already there, it was just not exposed via the Java DSL (GrpcStubMappingBuilder).

References

Solves #174

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[fcristovao]

Not sure if any documentation should be added to https://wiremock.org/docs/grpc/?