Allow admin extension to configure admin at initialize time

[yann-eugone]

Subject

Allow admin to be configured by extensions at initialize time.

I am targeting this branch, because code is backwards compatible.

Closes #6890.

Changelog

### Added
- Added `AdminExtensionInterface::configure` method
- Added `AbstractAdminExtension::configure` method with no content

### Changed
- Changed `AbstractAdmin::initialize` and call `AdminExtensionInterface::configure` on each extension

To do

• Update the documentation;

[dmaicher]

we could also add // NEXT_MAJOR: Uncomment this method

[VincentLanglet]

I will uncomment this during the merge

[rande]

@VincentLanglet @dmaicher if this is used to generate routes, this might lead to unexpected behaviour as Symfony will store the route in a cached file. This file will not change even if you add custom rules at runtime to alter routes.

[VincentLanglet]

I don't expect people to use it like this.
I assume it doesn't work either to override the configure method of the Admin class in order to generate routes ? So I would say, you can only do in the configure method of the AdminExtension the same thing you can do in the configure method of the Admin class.