API key client option

[cretz]

Describe the solution you'd like

See temporalio/features#426

[Quinn-With-Two-Ns]

Is AuthorizationGrpcMetadataProvider sufficient or do we need to add another way to set the API key?

example:

    // Create the Workflow service stub.
    WorkflowServiceStubsOptions.Builder stubOptions =
        WorkflowServiceStubsOptions.newBuilder()
            .setChannelInitializer(
                (channel) -> {
                  channel.overrideAuthority(SERVER_NAME);
                  channel.intercept(MetadataUtils.newAttachHeadersInterceptor(metadata));
                })
            .addGrpcMetadataProvider(
                new AuthorizationGrpcMetadataProvider(() -> "Bearer " + API_KEY))
            .setTarget(ENDPOINT);

[cretz]

I was hoping to have a literal setApiKey even if it's sugar, but can be convinced otherwise if we don't think it's needed here. I do think there may be a time soon where setCloudApiKey would default the target too, but that's for later.