Support for extracting response headers for unary calls

[6harat]

Is your feature request related to a problem? Please describe. We have a use-case where we want to simulate a unary call which returns context in its response headers which is required to be passed to subsequent unary calls. Based on our investigation, response Headers are not being captured in GrpcCallAction.ContinuingListener

    override def onHeaders(headers: Metadata): Unit = {}

And hence it would not be possible to extract that using this library without a code change.

Describe the solution you'd like A utility method exposed in GrpcCheckSupport which will allow extracting the desired response header

Describe alternatives you've considered No known alternatives found.

Additional context NA