[AAE-2718] List available roles for a group

[DenysVuika]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation
• Other... Please describe:

What is the current behaviour? (You can also link to an open issue here)

What is the new behaviour?

• introduce new OAuth2Service that unifies and simplifies custom api calls
• add missing "available roles" for the group service
• greatly cleanup Identity User and Group services, remove copy-pasted code
• remove useless error rethrowing (apps should handle that better)

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[codecov-commenter]

Codecov Report

Merging #5954 into develop will increase coverage by 0.60%.
The diff coverage is 75.45%.

@@             Coverage Diff             @@
##           develop    #5954      +/-   ##
===========================================
+ Coverage    76.08%   76.69%   +0.60%     
===========================================
  Files          843      835       -8     
  Lines        18769    18410     -359     
  Branches      3691     3619      -72     
===========================================
- Hits         14281    14120     -161     
+ Misses        3378     3191     -187     
+ Partials      1110     1099      -11     

Impacted Files	Coverage Δ
lib/core/services/identity-user.service.ts	67.18% <62.50%> (+10.47%)	⬆️
lib/core/services/identity-group.service.ts	68.42% <74.28%> (+1.75%)	⬆️
lib/core/mock/identity-group.service.mock.ts	100.00% <100.00%> (+11.11%)	⬆️
lib/core/services/oauth2.service.ts	100.00% <100.00%> (ø)
lib/extensions/src/test.ts
...b/extensions/src/lib/services/extension.service.ts
...sions/src/lib/services/extension-loader.service.ts
.../components/dynamic-component/dynamic.component.ts
...ons/src/lib/services/component-register.service.ts
lib/extensions/src/lib/services/rule.service.ts
... and 8 more

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