feat: Conditional execution of authorizers, contextualizers and unifiers in a rule

[dadrus]

Related issue(s)

closes #521

Checklist

• I agree to follow this project's Code of Conduct.
• I have read, and I am following this repository's Contributing Guidelines.
• I have read the Security Policy.
• I have referenced an issue describing the bug/feature request.
• I have added tests that prove the correctness of my implementation.
• I have updated the documentation.

Description

This PR implements the option 2 described in #521, which makes the following configuration possible:

- id: some_service_rule
  match:
    url: http://127.0.0.1:9090/foo/bar
  methods:
    - GET
    - POST
    - PUT
  execute:
    - authenticator: cookie_session
    - authenticator: anonymous
    # the next contextualizer executes only if the request is not anonymous
    - contextualizer: subscription_plan
      if: Subject.ID != "anonymous"
    # the next authorizer executes only if the request is an anonymous read request
    - authorizer: allow_all
      if: Subject.ID == "anonymous" && Request.Method == "GET"
    # the next authorizer executes only if the request is an anonymous write request
    - authorizer: deny_all
      if: Subject.ID == "anonymous" && (Request.Method == "POST" || Request.Method == "PUT")
    # the next authorizer executes only if the request is a write request
    - authorizer: can_write
      if: Request.Method == "POST" || Request.Method == "PUT" 
    - unifier: jwt_unifier

Conditional execution of unifiers is possible as well (can make the go templates less verbose if required)

[codecov]

Codecov Report

Merging #562 (5659df8) into main (7a6bc17) will decrease coverage by 0.12%.
The diff coverage is 87.71%.

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
- Coverage   89.33%   89.22%   -0.12%     
==========================================
  Files         210      213       +3     
  Lines        8915     8989      +74     
==========================================
+ Hits         7964     8020      +56     
- Misses        764      778      +14     
- Partials      187      191       +4     

Impacted Files	Coverage Δ
internal/rules/default_execution_condition.go	0.00% <0.00%> (ø)
internal/rules/cel_execution_condition.go	65.62% <65.62%> (ø)
internal/rules/rule_factory_impl.go	96.83% <98.61%> (+0.45%)	⬆️
internal/rules/conditional_subject_handler.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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