Add user-provided hook to accept/reject connections

[cBournhonesque]

Fixes #382
Fixes #381

• Add a field in the NetConfig to provide a closure Fn(ClientId) -> bool to decide if a connection will get accepted or rejected.
• Add a DeniedReason in the DeniedPacket, which will get showed to the client

TODO:
maybe instead we can pass in a separate object dyn ConnectionRequestHandler with

trait ConnectionRequestHandler {
   fn accept();
   
   fn reject() -> Reason
}

and the user can implement their own?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.26087% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 67.41%. Comparing base (e610332) to head (71ffdf2).

Files	Patch %	Lines
lightyear/src/connection/netcode/server.rs	64.28%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
+ Coverage   66.26%   67.41%   +1.15%     
==========================================
  Files         159      159              
  Lines       10338    10361      +23     
==========================================
+ Hits         6850     6985     +135     
+ Misses       3488     3376     -112     

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