enhance(backend): add unix socket support

[ybw2016v]

What

Add unix socket support, to make it possible to let misskey listen on the unix socket, which may improve backend performance.

Why

The unix socket is an inter-process communication mechanism and it can avoid some unnecessary checks and operations, which makes them faster and lighter than listening on the TCP/IP loopback port if the reverse proxy and misskey are running on the same machine.

Additional info (optional)

https://serverfault.com/questions/124517/what-is-the-difference-between-unix-sockets-and-tcp-ip-sockets

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

Merging #11275 (70fb71c) into develop (df54806) will decrease coverage by 0.12%.
The diff coverage is 43.75%.

@@             Coverage Diff             @@
##           develop   #11275      +/-   ##
===========================================
- Coverage    77.84%   77.73%   -0.12%     
===========================================
  Files          917      918       +1     
  Lines        92631    92703      +72     
  Branches      7769     7741      -28     
===========================================
- Hits         72112    72063      -49     
- Misses       20519    20640     +121     

Impacted Files	Coverage Δ
packages/backend/src/boot/master.ts	0.00% <0.00%> (ø)
packages/backend/src/server/ServerService.ts	48.61% <33.33%> (-0.98%)	⬇️
packages/backend/src/config.ts	96.46% <100.00%> (+0.03%)	⬆️

... and 18 files with indirect coverage changes

[syuilo]

👍🏻