Lift process watchdog as dedicated lib

This MR aims to lift the the mechanism that maintains the process of the RPC process alive (aka watchdog) into a dedicated library, for future use.

Do to so, the code was reworked to make it more generic -- thanks to several simple steps.

Then, it is move to the dedicated Lib_base/unix/lwt_process_watchdog.ml .

This MR is on top of !15324 (merged)

Manually testing the MR

This MR aims to be noop for the behaviour of the code base.

As it rework the event modules of the RPC process, one can check that the RPC process test are fine with TZ_SCHEDULE_KIND="EXTENDED_RPC_TESTS" dune exec tezt/tests/main.exe -- --file rpc_process.ml -j 4

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR