Workflow-init support

[drewhoskins-temporal]

We should have a method that's guaranteed to initialize before signals and updates come in so handlers can use initialized values.

[josh-berry]

see the parent feature issue for how other languages have handled it: temporalio/features#400

[roxblnfk]

I'm developing the concept of Workflow Init in the constructor: the user can call the Workflow::getInput() method to obtain input arguments.

    public function __construct() {
        $input = Workflow::getInput()->getValues();
        // ... init code
    }

However, getInput() returns a ValuesInterface, which does not contain type information.

Manually specifying the type for required arguments like Workflow::getInput()->getValue(0, DateTimeInterface::class); is inconvenient.
Additionally, new objects will be created instead of reusing existing ones.

My proposal is to add a new method Workflow::getTypedInput(), which will return the same arguments that will be sent to the Workflow Handler after initialization.
This way, we can provide a convenient Workflow Init using language mechanisms. At the same time, we still have the flexibility to add a separate InitHandler if needed.

[Sushisource]

@roxblnfk That works for me. If it's possible to allow them to specify the args in their constructor, and pass them in there I would say let's do that (possibly in addition to getTypedInput), possibly by using an annotation on the constructor like we've done for Python. If that's not feasible I think this approach will work just fine too.

[drewhoskins-temporal]

+1 to @Sushisource's comment, if it's doable. It seems somewhat more discoverable and idiomatic.