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?