feat: add option to not check build server source IP

[iTrooz]

Hello !

This PR adds an option to be able to disable the scheduler checking the IP of the build server when connecting.
While this option is useful in some cases, it can be annoying for more complex setups. For example, I am currently using sccache on servers behind a NAT that I do not control, and use port redirection services (like ngrok) to talk to the build servers.
This means that a build server does not have a single "public IP", but use a different IP when initiating or receiving connections.

Currently, I'm getting around that by adding a reverse-proxy in front of the scheduler, adding a fake X-Real-IP header, to force the scheduler to accept build server connections from any source IP. But I'd like to simplify my setup, hence this option

The default behaviour is unchanged

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.84615% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.36%. Comparing base (c0db872) to head (2b9f6d5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dist/http.rs	78.33%	13 Missing ⚠️
src/bin/sccache-dist/main.rs	0.00%	2 Missing ⚠️
src/config.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2709      +/-   ##
==========================================
+ Coverage   74.34%   74.36%   +0.02%     
==========================================
  Files          70       70              
  Lines       39373    39440      +67     
==========================================
+ Hits        29271    29329      +58     
- Misses      10102    10111       +9     

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[sylvestre]

please update the doc and also add a test to check this (esp in case of mismatch)
thanks