discoveryengine: add list resources

[modular-magician]

What this PR does

Enables MMv1 list resource generation for Discovery Engine MMv1 resources by adding generate_list_resource: true to each eligible resource YAML.

Also adds a new list_filter YAML field (and template support) to allow list resources to append a static ?filter= query parameter — used by the three engine resources which share the same API endpoint and are distinguished by solutionType.

Discovery Engine resources with list support added

• google_discovery_engine_assistant
• google_discovery_engine_chat_engine
• google_discovery_engine_cmek_config
• google_discovery_engine_control
• google_discovery_engine_data_store
• google_discovery_engine_license_config
• google_discovery_engine_recommendation_engine
• google_discovery_engine_schema
• google_discovery_engine_search_engine
• google_discovery_engine_serving_config
• google_discovery_engine_sitemap
• google_discovery_engine_target_site
• google_discovery_engine_user_store

Resources excluded

• google_discovery_engine_data_connector — singleton resource (no list API)
• google_discovery_engine_acl_config — singleton resource (no list API)
• google_discovery_engine_widget_config — list API only exists at v1alpha; product base path is v1

Notes

• ChatEngine, SearchEngine, and RecommendationEngine all share projects.locations.collections.engines as their API endpoint, differentiated by solutionType. The new list_filter field generates ?filter=solution_type=SOLUTION_TYPE_* on each list call so results are scoped to the correct engine type.
• Sitemap uses a :fetch endpoint returning a sitemapsMetadata nested structure; collection_url_key: sitemapsMetadata and an updated post_read template handle both the list and single-resource read contexts correctly.
• Control and ServingConfig have collection_id with default_value: default_collection. The list method template now seeds default_value into resourceData when the caller omits the field, matching the resource schema Default: behaviour.

`google_discovery_engine_assistant`

`google_discovery_engine_chat_engine`

`google_discovery_engine_cmek_config`

`google_discovery_engine_control`

`google_discovery_engine_data_store`

`google_discovery_engine_license_config`

`google_discovery_engine_recommendation_engine`

`google_discovery_engine_schema`

`google_discovery_engine_search_engine`

`google_discovery_engine_serving_config`

`google_discovery_engine_sitemap`

`google_discovery_engine_target_site`

`google_discovery_engine_user_store`

Derived from GoogleCloudPlatform/magic-modules#18231