Shell/RPC: allow to monitor heads only of the current protocol
Context
Some daemons like the baker need to monitor blocks whose next protocol is theirs so they can, e.g., build the following block. Other applications (like the rollup nodes) need to instead monitor blocks whose content is in the current protocol in order to react to it.
This MR adds a query parameter protocols to the streamed RPC ../monitor/heads in order to select blocks of which protocol we want to see in the stream.
This MR is one of the preliminary steps for the rollup node to support protocol migrations (#2901 (closed)).