feat(forms): add validateAsync overloads for simple function and resource options - #70124

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

• validateAsync()lacked full overload definitions to support both lightweight async validation logic (similar to synchronous validate() and direct Angular resource() integration

Issue Number: #69779

What is the new behavior?

• Adds public API overloads and implementation logic for validateAsync():
  • Simple function overload: Accepts a lightweight async validator function alongside an optional configuration object (debounce, when, onError).
  • Resource object overload: Accepts an AsyncValidatorOptions configuration for direct integration with Angular's resource() primitive (params, factory, onSuccess, onError, debounce, and when)
  • Integrates resource-based validation rules into field metadata so async validation executes automatically once all synchronous validation passes.

Does this PR introduce a breaking change?

• Yes
• No

Other information

• Backward compatible and preserves all existing functionality without breaking preexisting async validation APIs like validateHttp()

• Supports parameter debouncing via DebounceTimer and signal chaining for reactive updates