Feature/json schema improvements

[Anticom]

Improve the current JSON schema implementation by the following:

1. Add support for passing a backed enum to Type's enum method.
   Before:

   $schema->string('oneof')->enum(['a', 'b', 'c']);

   After:

   enum Options: string
   {
     case A = 'a';
     case B = 'b';
     case C = 'c';
   }
   
   $schema->string('oneof')->enum(Options::class);

2. Add missing format for StringType.
   New:

   $schema->string('somedate')->format('date');

I didn't create an issue beforehand since I deemed those additions so minor.

The improved enum handling just makes sense looking at how Laravel supports this in various other places.
The missing format was added primarily to improve laravel/mcp.
Ref.: https://modelcontextprotocol.io/docs/learn/server-concepts#how-tools-work

Tests have been added.

Cheers,
Anticom