Allow configuration of per-provider publishers policy

[gammazero]

This allows each provider to have its own policy for which publishers are allowed to publish advertisements on behalf of that provider. The previous publisher policy serves as the default when there is no policy configured for a specific provider.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (e7299f0) 44.68% compared to head (02024c8) 44.81%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/registry/policy/policy.go	62.06%	8 Missing and 3 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2526      +/-   ##
==========================================
+ Coverage   44.68%   44.81%   +0.12%     
==========================================
  Files          87       87              
  Lines       11007    11031      +24     
==========================================
+ Hits         4919     4943      +24     
+ Misses       5497     5494       -3     
- Partials      591      594       +3     

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[gammazero]

Note: This leaves the default publisher policy config (Discovery.Policy.Publish = true) to allow any peer to publish/sign advertisements for a provider that does not have its own specific publisher policy. If we want to make the default behavior to block any publisher (with peerID different from provider), that is not explicitly allowed, then this should be changed to false.