feat(sysctl): Add helper command

[msbrogli]

Acceptance criteria

1. Add !help command, which shows all commands available.
2. Add !help [cmd] command, which shows a help message for the command.

Example

!help
p2p.always_enable_sync
p2p.always_enable_sync.readtxt
p2p.force_sync_rotate
p2p.max_enabled_sync
p2p.rate_limit.global.send_tips
p2p.sync_update_interval
!help p2p.always_enable_sync
getter() -> List[str]:
    Return the list of sync-always-enabled peers.

setter(values: List[str]) -> None:
    Change the list of sync-always-enabled peers.
!help p2p.rate_limit.global.send_tips
getter() -> Tuple[int, float]:
    Return the global rate limiter for SEND_TIPS.

setter(max_hits: int, window_seconds: float) -> None:
    Change the global rate limiter for SEND_TIPS.

    The rate limiter is disabled when `window_seconds == 0`.

[codecov]

Codecov Report

Merging #620 (d7508e0) into master (9ba84cc) will decrease coverage by 0.02%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #620      +/-   ##
==========================================
- Coverage   83.62%   83.61%   -0.02%     
==========================================
  Files         232      232              
  Lines       19788    19826      +38     
  Branches     2707     2714       +7     
==========================================
+ Hits        16548    16577      +29     
- Misses       2640     2647       +7     
- Partials      600      602       +2     

Impacted Files	Coverage Δ
hathor/sysctl/protocol.py	84.31% <85.71%> (+0.25%)	⬆️
hathor/sysctl/sysctl.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes