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[TwigBridge] added a render_* function to ease usage of custom rendering strategies

Here is the code you need to write when using the regular render
function for an ESI strategy:

    {{ render(path('path'), { strategy: 'esi' })
}}

And the same with the new render_* function:

    {{ render_esi(path('path')) }}
[HttpKernel] added a URL signer mechanism for hincludes
[HttpKernel] added missing phpdoc and tweaked existing ones
[HttpKernel] simplified and enhanced code managing the hinclude strategy
made some services private