Add support for CallCredentials in akka-http backend

[dwhjames]

Merge metadata generated by CallCredentials callback into headers at request time.

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I’m putting up this PR as WiP to get feedback from maintainers.

[dwhjames]

@raboof,

The Netty-based client has limited support for CallOptions, but it does currently support the call credentials feature.

akka-grpc/codegen/src/main/twirl/templates/ScalaClient/Client.scala.txt

Line 56 in 5c0f851

private val options = NettyClientUtils.callOptions(settings)

akka-grpc/runtime/src/main/scala/akka/grpc/internal/NettyClientUtils.scala

Lines 194 to 198 in 5c0f851

	/**
	* INTERNAL API
	*/
	@InternalApi def callOptions(settings: GrpcClientSettings): CallOptions =
	settings.callCredentials.map(CallOptions.DEFAULT.withCallCredentials).getOrElse(CallOptions.DEFAULT)

I was hoping to do the same for the akka-http based client.

When you have a chance, I would welcome any feedback on extending AkkaHttpClientUtils.scala