MINOR: tcp-act: parse 'tcp-request attach-srv' session rule

MINOR: tcp-act: parse 'tcp-request attach-srv' session rule

Create a new tcp-request session rule 'attach-srv'.

The parsing handler is used to extract the server targetted with the
notation 'backend/server'. The server instance is stored in the act_rule
instance under the new union variant 'attach_srv'.

Extra checks are implemented in parsing to ensure attach-srv is only
used for proxy in HTTP mode and with listeners/server with no explicit
protocol reference or HTTP/2 only.

The action handler itself is really simple. It assigns the stored server
instance to the 'reverse' member of the connection instance. It will be
used in a future patch to implement passive reverse-connect.