Remove NSString API usage

[Cyberbeni]

These changes are now available in 5.4.1

resolves #242

Remove NSString API usage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.19%. Comparing base (aa60a21) to head (a8928c5).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
- Coverage   82.21%   82.19%   -0.02%     
==========================================
  Files          61       61              
  Lines        1456     1455       -1     
==========================================
- Hits         1197     1196       -1     
  Misses        259      259              

Files with missing lines	Coverage Δ
Sources/JWTKit/Utilities/Base64URL.swift	100.00% <100.00%> (ø)

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[ptoffy]

Actually this won't work in iOS 15. We have a native base64URL unescaping function already, let's use it

extension String {
    package func base64URLDecodedData() -> Data? {
        var data = Data(self.utf8)
        data.base64URLUnescape()
        return Data(base64Encoded: data)
    }
}