MEDIUM: Add tcp-request switch-mode action to perform HTTP upgrade

MEDIUM: Add tcp-request switch-mode action to perform HTTP upgrade

It is now possible to perform HTTP upgrades on a TCP stream from the
frontend side. To do so, a tcp-request content rule must be defined with the
switch-mode action, specifying the mode (for now, only http is supported)
and optionnaly the proto (h1 or h2).

This way it could be possible to set HTTP directives on a TCP frontend which
will only be evaluated if an upgrade is performed. This new way to perform
HTTP upgrades should replace progressively the old way, consisting to route
the request to an HTTP backend. And it should be also a good start to remove
all HTTP processing from tcp-request content rules.

This action is terminal, it stops the ruleset evaluation. It is only
available on proxy with the frontend capability.

The configuration manual has been updated accordingly.