Refactor Provider classes

[tpazderka]

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.

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Touch of refactoring, Updated Provider classes.

[codecov-io]

Codecov Report

Merging #628 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
+ Coverage   61.51%   61.63%   +0.11%     
==========================================
  Files          62       62              
  Lines       11140    11121      -19     
  Branches     1973     1967       -6     
==========================================
+ Hits         6853     6854       +1     
+ Misses       3727     3709      -18     
+ Partials      560      558       -2

Impacted Files	Coverage Δ
src/oic/oic/provider.py	65.85% <ø> (+0.75%)	⬆️
src/oic/oauth2/provider.py	70.26% <100%> (ø)	⬆️
src/oic/utils/http_util.py	68.67% <0%> (+0.57%)	⬆️
src/oic/utils/authn/user.py	64.22% <0%> (+0.81%)	⬆️

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[tpazderka]

Part of #629

[schlenk]

LGTM