Make each handler type have its own extension class

[GromNaN]

Q	A
Branch?	3.x
Bug fix?	no
New feature?	yes
Deprecations?	no (not yet)
Issues	Fix #558
License	MIT

This is a partial PR to gather feedbacks.

• Each Monolog handler has its own configuration and extension class
• Some handlers can share the same with different properties (name, handler class)
• They are all registered in the MonologExtension
• 3rd party bundles can add their own handler extension classes

[mbabker]

• depending on where the Configuration class is accessed, the extensions might be registered or no (we have that issue in SecurityBundle, which is the only bundle I'm aware of doing such thing

knplabs/knp-gaufrette-bundle, liip/imagine-bundle, payum/payum-bundle, and sylius/theme-bundle all do something similar

[ThomasLandauer]

Could this also be used to pass the handler's FQCN as type, rather than some string?:

'handlers' => [
    'foobar' => [
        'type' => StreamHandler::class