This PR was merged into the 4.3-dev branch.

Discussion
----------

[DI] Add an url EnvProcessor

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#11128

This PR add a new env processor `url` to convert an URL (or DSN) into an array.

The main goal is to simplify the project configuration and reduce the number of env variable when working with bundle which are not able to deal with DSN
(pick some random project in symfony/recipes-contrib: https://github.com/symfony/recipes-contrib/blob/master/facile-it/mongodb-bundle/0.6/manifest.json or https://github.com/symfony/recipes-contrib/blob/master/wouterj/eloquent-bundle/1.0/manifest.json)

```yaml
# before
MONGODB_HOST=localhost
MONGODB_PORT=27017
MONGODB_USER=
MONGODB_PASSWORD=
MONGODB_DB=symfony

mongo_db_bundle:
    data_collection: '%kernel.debug%'
    clients:
        default:
            hosts:
            - { host: '%env(MONGODB_HOST)%', port: '%env(int:MONGODB_PORT)%' }
            username: '%env(MONGODB_USER)%'
            password: '%env(MONGODB_PASSWORD)%'
            connectTimeoutMS: 3000
    connections:
        default:
            database_name: '%env(MONGODB_DB)%'

# after
MONGODB_DSN=mongodb://localhost:27017/symfony

mongo_db_bundle:
    data_collection: '%kernel.debug%'
    clients:
        default:
            hosts:
            - { host: '%env(key:host:url:MONGODB_DSN)%', port: '%env(key:port:url:MONGODB_DSN)%' }
            username: '%env(key:user:url:MONGODB_DSN)%'
            password: '%env(key:pass:url:MONGODB_DSN)%'
            connectTimeoutMS: 3000
    connections:
        default:
            database_name: '%env(key:path:url:MONGODB_DSN)%'
```

Added also a `query_string` processor to parse query string

```
DATABASE_DSN=mysql://localhost/db?charset=utf8

foo:
  bar:
    charset: '%env(key:charset:query_string🔑query:url:DATABASE_DSN)%'
```

Commits
-------

f253c9b7ca Add an url EnvProcessor