Global resource defaults implementation

[bpolaszek]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#3099
License	MIT

This PR introduces a defaults configuration as a new implementation of #3099 (#3135 was dropped).

This one doesn't rely on a low-priority DefaultsResourceMetadataFactory as suggested first (because low priority means late access to some normalization logic) and implements the defaults pattern in AnnotationResourceMetadataFactory and ExtractorResourceMetadataFactory, making configuration more consistent, which means support for short-hand config like this:

api_platform:
    defaults:
        itemOperations: ["get"]
        collectionOperations: ["get"]
        mercure: true

Default configuration is now flattened at the bundle configuration level, removing the attributes sublevel to improve DX and normalizing all keys to snake_case to follow Symfony standards (both camelCase and snake_case are actually supported to be less error-prone, attributes being mostly documented as annotations):

api_platform:
    defaults:
        description:          ~
        iri:                  ~
        item_operations:      ~
        collection_operations: ~
        graphql:              ~
        security:             ~
        security_message:     ~
        security_post_denormalize: ~
        security_post_denormalize_message: ~
        cache_headers:        ~
        denormalization_context: ~
        deprecation_reason:   ~
        elasticsearch:        ~
        fetch_partial:        ~
        force_eager:          ~
        formats:              ~
        filters:              ~
        hydra_context:        ~
        mercure:              ~
        messenger:            ~
        normalization_context: ~
        order:                ~
        pagination_client_enabled: ~
        pagination_client_items_per_page: ~
        pagination_client_partial: ~
        pagination_enabled:   ~
        pagination_fetch_join_collection: ~
        pagination_items_per_page: ~
        maximum_items_per_page: ~
        pagination_maximum_items_per_page: ~
        pagination_partial:   ~
        pagination_via_cursor: ~
        route_prefix:         ~
        swagger_context:      ~
        validation_groups:    ~
        sunset:               ~
        input:                ~
        output:               ~
        openapi_context:      ~

[soyuka]

Couldn't we add a DefaultMetadataFactory that is called prior to extractor/annotation that sets defaults based on the injected defaults?
#3135 (comment)

I'm not fond of the transformations done in the Extension.

[dunglas]

Thanks @bpolaszek!