BUGFIX: Stability and performance improvements

[bwaidelich]

• Don't fetch JWKs for every request (JWKs are fetched for every (authentication) request #9)
• Don't share discovery cache over multiple services (options cache is shared between all configured services #10)
• Don't require options before they are used (clientId and clientSecret have to be configured even if they are not required #11)

[codecov]

Codecov Report

Merging #12 into master will increase coverage by 5.71%.
The diff coverage is 58.33%.

@@             Coverage Diff              @@
##             master      #12      +/-   ##
============================================
+ Coverage      6.75%   12.47%   +5.71%     
- Complexity      175      176       +1     
============================================
  Files             9        9              
  Lines           459      457       -2     
============================================
+ Hits             31       57      +26     
+ Misses          428      400      -28     

Impacted Files	Coverage Δ	Complexity Δ
Classes/Authentication/OpenIdConnectEntryPoint.php	0.00% <0.00%> (ø)	11.00 <0.00> (ø)
Classes/OpenIdConnectClient.php	25.53% <60.86%> (+25.53%)	40.00 <10.00> (+1.00)
Classes/OAuthClient.php	7.40% <0.00%> (+7.40%)	15.00% <0.00%> (ø%)

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