[Feature Request] API key client option

[cretz]

Describe the solution you'd like

See temporalio/features#426 and temporalio/sdk-go#1409 for the ideal approach

[roxblnfk]

Since PHP SDK 2.9.0 users can specify any metadata, including the Authorization header.

/** @var \Temporal\Client\WorkflowClient $workflowClient */

$workflowClient = $workflowClient->withMetadata(['Authorization', ['Bearer <api-key>']]);

// All the calls $workflow->* will be executed with the specified metadata.
$workflowClient->newWorkflowStub(MyWorkflow::class);

Do we really need to add extra sugar for this header?

[josh-berry]

I think it's a nice thing to have ergonomically, and it matches what the other SDKs do (or will do). So unless it's gonna cause some significant amount of ugliness, I personally think it's worth doing.