Add a Symfony client factory

[Nyholm]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Related tickets
Documentation
License	MIT

This PR is blocked by #372. It will allow you to configure Symfony clients like:

httplug:
    clients:
        default:
            factory: 'httplug.factory.symfony'
            http_methods_client: true
            config:
                timeout: 4  # Symfony config
            plugins:
                - 'httplug.plugin.logger'

[dbu]

supporting the symfony client definitely makes sense, yes.

doesn't symfony provide its http client as a service already? why do we need our own factory?

[Nyholm]

We want to integrate with HTTPlug ecosystem, plugins, cache, bundle support etc.

[Nyholm]

I've rebased the PR and the tests are green

[XWB]

Looks good.

Please merge and tag a new release please :)