Configurable asset package

[smatyas]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tickets	fixes #3760
License	MIT
Doc PR	api-platform/docs#1186

This PR introduces a new config parameter: asset_package

It enables the users to specify a custom asset package (as in Symfony framework) to be used by the Swagger UI template.
In this way, we have more fine-grained control over the asset url generations.

The new parameter is optional with a null default value. It results in the same asset(...) twig function calls as it was before.

---

Example usage of the new parameter:

# config/packages/api_platform.yaml
api_platform:
    asset_package: 'api_platform'

and the corresponding symfony framework setup example:

# config/packages/framework.yaml
framework:
    # ...
    assets:
        base_path: '/custom_base_path' # the default
        packages:
            api_platform:
                base_path: '/'

[smatyas]

The cs-fixer test seems to be invalid, the diff does not contain the failing change anymore.

[soyuka]

thanks @smatyas !