[9.x] Adds enum casting to Request

[emargareten]

(same as #43169 for 9.x)

This PR allows to conveniently retrieve an input from the Request as an enum.

// Before
public function post(Request $request)
{
    $status = StatusEnum::tryFrom($request->input('status'));

    // do stuff with status enum...
}

// After
public function post(Request $request)
{
    $status = $request->enum('status', StatusEnum::class);

    // do stuff with status enum...
}

[supun-io]

I just added a PR to improve type definitions of this.

By the way, I was thinking, shouldn't there be a way to get non-nullable Enum?

For example, if I validate the input as an enum, I know it is a valid Enum value later.

$request->validate(['type' => ['required', new Enum(MyEnum::class)]);

$type = $request->enum('type', MyEnum::class);
// currently this is MyEnum | null

Currently, $type can be null because tryFrom is used. But, as we have validated earlier, we may just need the exact enum. It is annoying to deal with null.

I had two ideas.

1. Add a third-parameter to the enum method.

public function enum($key, $enumClass, $nullable = true) {}

Then, call it as $request->enum($key, $enumClass, false). But, this is not very fluent.

2. Add a different method

As this is already deployed to public 9.0, we will need to reserve the enum name for nullable method. I am not sure about a name for the other method. enumNotNullable is too long.

Any ideas? @emargareten