Add key removal functions to JWTKeyCollection

[ptoffy]

As discussed in #239, key removal functions can be helpful to implement key rotation. A key rotation example will follow.
APIs:

• clearDefault(): removes the default signer (might still be in the collection though not as default, so its kid must be specified to use it)
• removeAll(except:, clearingDefault:): removes all signers besides the ones specified in the list and also clears the default if the parameter set to true
• remove(kid:): removes the specified kid from the list (without considering the default)

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.21%. Comparing base (2681975) to head (f00a7ae).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/JWTKit/JWTKeyCollection.swift	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   82.00%   82.21%   +0.20%     
==========================================
  Files          61       61              
  Lines        1434     1456      +22     
==========================================
+ Hits         1176     1197      +21     
- Misses        258      259       +1     

Files with missing lines	Coverage Δ
Sources/JWTKit/JWTKeyCollection.swift	93.57% <95.45%> (+0.47%)	⬆️

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[0xTim]

@ptoffy some doc clarifications still needed here