Add availability annotations to unavailable crypto properties

[ptoffy]

Since adding support for iOS 15, builds started failing at https://swiftpackageindex.com/builds/495EA704-A76E-4078-BAA9-617D0769824E because of an unavailable property (namely compressedRepresentation) which is available from iOS 16 on. This adds annotations to make the property unavailable to iOS 15 (and therefore using Crypto's implementation)

Fixes #191

[0xTim]

@ptoffy looks like my iOS tests aren't working correctly - this should be failing on main. I think I picked an OS that isn't supported judging by the build log.

Could you remove the fixes, get CI to fail and then re-add them?

[ptoffy]

Cool @0xTim we should be good

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.28%. Comparing base (3a09049) to head (ec913ba).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #192   +/-   ##
=======================================
  Coverage   83.28%   83.28%           
=======================================
  Files          60       60           
  Lines        1424     1424           
=======================================
  Hits         1186     1186           
  Misses        238      238           

Files with missing lines	Coverage Δ
Sources/JWTKit/ECDSA/ECDSAKeyTypes.swift	100.00% <ø> (ø)