Add kid param to sign function

[ptoffy]

This adds back the kid parameter to the sign function, since passing it in the header looks a bit funny:

try await keyCollection.sign(somePayload, header: ["kid": .string(someKid.string)])

This can now be rewritten as

try await keyCollection.sign(somePayload, kid: someKid)

The passed in kid is prioritised, while the header is searched if none is passed in.
Also removes a now useless @preconcurrency attribute

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.05%. Comparing base (65eb071) to head (91ab500).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   68.85%   69.05%   +0.20%     
==========================================
  Files          59       59              
  Lines        1554     1561       +7     
==========================================
+ Hits         1070     1078       +8     
+ Misses        484      483       -1     

Files	Coverage Δ
Sources/JWTKit/JWTKeyCollection.swift	89.33% <100.00%> (+1.09%)	⬆️
Sources/JWTKit/RSA/RSA.swift	75.00% <ø> (ø)

... and 1 file with indirect coverage changes