feat(forms): add custom equality function for signal forms - #70299

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?

Signal Forms currently use reference equality for object values. If a new object instance represents the same logical value, the form still treats it as a change and updates dependent signals

Issue Number: #69928

What is the new behavior?

Adds a custom equality() rule that allows callers to define custom value comparison logic for fields:

• Fields can opt into custom equality checks.
• Values that compare as equal no longer replace the current field value.
• Dependent signals are not notified when the comparator reports no change.
• Custom equality logic is consistently applied across nested fields, array items, conditional rules, and deepSignal updates.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Extracted the shared, dependency-free metadata primitives (MetadataKey, MetadataReducer, createMetadataKey, IS_ASYNC_VALIDATION_RESOURCE, LimitKey, LimitSelectionKey, MetadataSetterType) into a new schema/metadata.ts module. This decoupling is required (imo) because the new EQUALITY MetadataKey needs to be read directly within the schema layer (schema/logic_node.ts) to retrieve the configured equality function during change detection