Respect CastAttributes::castUsing generic types

[Watercycle]

Summary

See #1700

This PR adds some logic for parsing generic types from CastAttributes::castUsing doc-blocks to allow for specializing castUsing while casting Laravel model columns.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• Existing tests have been adapted and/or new tests have been added
• Update the README.md
• Code style has been fixed via composer fix-style

[Watercycle]

As a general disclaimer: This probably doesn't work with templated generic types (e.g. @template TGet) and does not solve this related issue: #1437. If you have a more generic solution in mind or see this as fundamentally flawed, please let me know and I'll take another pass at this!