Add field type extensions

[saleyn]

This change enables to extend fields with custom types in order to be able to inject custom body in a field wrapper.

[greptile-apps]

PR Summary

This PR introduces a field extension behavior system that allows developers to create custom field types with their own rendering logic and CSS classes in the Petal Components library.

• Added new PetalComponents.Field.Extension behavior with required render/1 and optional get_class_for_type/1 callbacks
• Implemented persistent term storage for field extensions to optimize performance (6.96x faster than Application.get_env)
• Added support for merging custom assigns with field extensions for flexible customization
• Added comprehensive test coverage for extension fields with datalist support
• Introduced phx-debounce support for form validation with configurable delay

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