Add new key addition API

[ptoffy]

#165 noted that the key addition API isn't conform to the Swift style guidelines, this fixes that

[0xTim]

We can roll out a new beta when merged

[gwynne]

@ptoffy Please fix the CI 🤨

[codecov]

Codecov Report

Attention: Patch coverage is 63.63636% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 77.65%. Comparing base (c4828b0) to head (5cb99a8).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   73.56%   77.65%   +4.09%     
==========================================
  Files          59       60       +1     
  Lines        1615     1392     -223     
==========================================
- Hits         1188     1081     -107     
+ Misses        427      311     -116     

Files	Coverage Δ
Sources/JWTKit/ECDSA/JWTKeyCollection+ECDSA.swift	100.00% <ø> (ø)
Sources/JWTKit/EdDSA/JWTKeyCollection+EdDSA.swift	100.00% <ø> (ø)
Sources/JWTKit/RSA/JWTKeyCollection+RSA.swift	85.71% <ø> (+52.38%)	⬆️
Sources/JWTKit/HMAC/HMAC.swift	75.00% <75.00%> (ø)
Sources/JWTKit/HMAC/JWTKeyCollection+HMAC.swift	75.00% <33.33%> (+41.66%)	⬆️

... and 26 files with indirect coverage changes