feat: Add JWT leeway setting to KeycloakOAuth2 backend

[mageo]

Fixes #1102 by adding support for a JWT_LEEWAY setting in the Keycloak backend.

[nijel]

I'd really prefer to have a generic solution here instead of ad-hoc solutions per backend. See also #1031

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.69%. Comparing base (326d4b3) to head (c7ba3ad).
Report is 150 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1103      +/-   ##
==========================================
- Coverage   77.88%   77.69%   -0.20%     
==========================================
  Files         347      348       +1     
  Lines       10669    10636      -33     
  Branches      504      461      -43     
==========================================
- Hits         8310     8264      -46     
- Misses       2200     2212      +12     
- Partials      159      160       +1     

Flag	Coverage Δ
unittests	77.69% <ø> (-0.20%)	⬇️

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