Feature: resource default config

[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 an implementation of #3099.

Dummy implementation
As discussed offline with @alanpoulain and @dunglas, the implementation has been thought to be as dummy as possible, i.e. any configurable property or attribute in @ApiResource (and even future ones) can be configured on a default basis.

This means this configuration is perfectly correct for all resources:

api_platform:
    defaults:
        attributes:
            mercure: true
            messenger: true

But this one too:

api_platform:
    defaults:
        description: foo

Which means unconfigured @ApiResources will have the same description. That doesn't make sense, but we assume it's up to the developer, not the implementation, to decide what makes sense and what doesn't.

Short-hand operations configuration aren't supported yet
This short-hand configuration:

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

is currently normalized in a private method from another ResourceMetadataFactory with a higher priority. What are your recommendations to standardize it?

[bpolaszek]

As discussed offline with @dunglas, several issues may be raised with this approach:

• Defaults should be flattened (no more attributes sublevel)
• snake_case should be the used
• Using a decorating service with a low priority may prevent it from having the right configuration, as some data is normalized before.
• Some properties should indeed be prevented from having defaults, e.g. shortName.

I'll open another PR to reflect those changes.