feat: consumer hooks to scope field uniqueness + visibility picker

[ManukMinasyan]

Summary

Adds two opt-in consumer hooks so an application can scope custom-field management to a subset of an entity's fields, instead of the package's default "all fields are global per entity_type" behaviour. Both hooks are strict no-ops when not registered, so existing consumers are unaffected.

This is needed by consumers that build multiple separate forms on one entity type (e.g. a versioned form builder), where the same field names/sections are intentionally reused across forms. Today such consumers hit two problems:

1. Field-name uniqueness is entity-type-wide. Two forms can't both have a field named "Address" — editing one throws "The name has already been taken", even though they belong to different forms.
2. The conditional-visibility "depends on" picker lists every field of the entity type. A field's visibility condition dropdown shows duplicate-named fields from every other form, so authors can't tell which one to pick.

The section-level analog of (1) already had a hook (ManageCustomFieldSection::resolveUniqueRuleModifierUsing + SectionForm::modifyUniqueRuleUsing). This PR adds the matching field-level hook and a visibility-picker scope hook.

What's added

• FieldForm::resolveUniqueRuleModifierUsing(?Closure) — register a resolver fn (?CustomFieldSection $section): ?Closure that returns a Unique rule modifier (or null). Applied to the name uniqueness rule only (code remains globally unique, consistent with the package's code-based field resolution). FieldForm::schema() gains an optional ?CustomFieldSection $section param, passed by ManageCustomField::editAction() (the edited field's section) and ManageCustomFieldSection::createFieldAction() (the create-target section).
• VisibilityComponent::resolveAvailableFieldsScopeUsing(?Closure) — register a resolver fn (string $entityType, ?CustomFieldSection $section): ?Closure that returns a query constraint (or null) applied to getAvailableFields(). make() / makeForSection() gain an optional ?CustomFieldSection so the field/section being edited can be passed through (FieldForm → field-level conditions, SectionForm::scopeVisibilityToSection() → section-level conditions).

Backward compatibility

• No resolver registered → identical behaviour to before (verified by tests).
• make() / schema() / makeForSection() new params are optional and default to null.
• Only the field name rule is scoped; code stays globally unique.

Tests

tests/Feature/ConsumerScopeHooksTest.php:

• VisibilityComponent lists all entity fields with no resolver (back-compat), constrains to the resolver subset when registered, and falls back to all when the resolver returns null.
• FieldForm::schema() builds unchanged with no resolver and accepts an optional section.

Exercised end-to-end by the consuming application (a versioned Flex Fund Form builder): editing a field whose name exists in another form now saves, and the visibility dropdown lists only the current form's fields.