Create metadata mapping form and service for migration

Problem/Motivation

As a site administrator,
I want to map Drupal media field values to Acquia DAM metadata fields so that dynamic metadata is applied at upload time,
so that migrated assets carry complete metadata (keywords, captions, custom fields) without a second metadata-sync pass.

Acceptance Criteria:

• Mapping UI (wizard step): AcquiaDamMigrationMetadataForm (between MapForm and SummaryForm) renders one static mapping table per included core media type with a single row per Drupal field ({ Drupal field (static label) | DAM metadata field select }); no add/remove AJAX machinery; non-included types get an italic gray note; stored mappings seed the select defaults (getStoredMappingByField()); empty allowed_metadata shows a notice linking to /admin/config/acquia-dam/metadata
• Drupal field filtering: only content fields are offered — the media source file field, DAM-managed fields (acquia_dam_asset_id / acquia_dam_managed_file / acquia_dam_managed_image) and base/read-only fields are excluded via the shared parent service acquia_dam.media_type_field_options (MediaTypeFieldOptions, also used by MediaTypeFormAlter); image media types additionally get a synthetic alt_text entry backed by MigrationMetadataMapper::getAltTextValues() ) reading the source field's alt
• Mapping storage: metadata_mapping section in acquia_dam_migration_tools.migration_preferences (stored_values) — sequence of { media_type_id, media_type_label, mappings: [{drupal_field, dam_field}] }; the Drupal field is the form row key (static column), so normalizeMetadataMapping() keys on it and drops rows with an empty dam_field; config schema updated
• Wizard wiring: AcquiaDamMigration ← updated (3-page wizard: 0 Map, 1 Metadata, 2 Summary); AcquiaDamMigrationSummaryForm ← updated (metadata_mapping_info summary table); MigrationTrait ← updated (isImageSource(), normalizeMetadataMapping(), getDrupalMetadataFieldOptions())