fix(Keycloak): adds missing HTTP Header to be used in the code-to-token request

[intellitour]

Fixes #80

[fpseverino]

Thanks, LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.41%. Comparing base (89b8b66) to head (462ed83).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
+ Coverage   82.37%   82.41%   +0.04%     
==========================================
  Files          59       59              
  Lines         800      802       +2     
==========================================
+ Hits          659      661       +2     
  Misses        141      141              

Files with missing lines	Coverage Δ
Sources/ImperialKeycloak/KeycloakRouter.swift	100.00% <100.00%> (ø)

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