refactor!: mutator renamed to unifier

[dadrus]

This PR finalizes the series consisting of #370 and #374 and finally closes #28.

The reason for this PR is that all Zero Trust papers are talking about unification of a request, respectively subject information - unifying the collected information into a format expected and understood by the upstream service. The name "mutator", respectively the verb "mutation", which was so far used by heimdall for the same purpose, even if it talks about unification in the concept description, was misleading and not understood by many people.

Old:

# heimdall config (e.g. heimdall.yaml)

rules:
  mechanisms:
    # other mechanism definitions
    mutators:
      - id: foo
        type: <specific type>
        config:
          <type specific config>
    #  other mechanism definitions

# rule file (e.g. rules.yaml)

- id: foo
  # other properties
  execute:
    # other mechanisms
    - mutator: foo
    # other mechanisms

New:

# heimdall config (e.g. heimdall.yaml)

rules:
  mechanisms:
    # other mechanism definitions
    unifiers:  # <-  changed
      - id: foo
        type: <specific type>
        config:
          <type specific config>
    #  other mechanism definitions

# rule file (e.g. rules.yaml)

- id: foo
  # other properties
  execute:
    # other mechanisms
    - unifier: foo   # <- changed
    # other mechanisms

[codecov]

Codecov Report

Merging #375 (8a2c49d) into main (f20bc37) will increase coverage by 0.09%.
The diff coverage is 92.17%.

@@            Coverage Diff             @@
##             main     #375      +/-   ##
==========================================
+ Coverage   86.26%   86.35%   +0.09%     
==========================================
  Files         193      193              
  Lines        7738     7727      -11     
==========================================
- Hits         6675     6673       -2     
+ Misses        907      901       -6     
+ Partials      156      153       -3     

Impacted Files	Coverage Δ
...ternal/rules/mechanisms/unifiers/config_decoder.go	78.57% <ø> (ø)
...ternal/rules/mechanisms/unifiers/cookie_unifier.go	83.33% <85.71%> (ø)
...ternal/rules/mechanisms/unifiers/header_unifier.go	83.33% <85.71%> (ø)
...rules/mechanisms/unifiers/unifier_type_registry.go	87.50% <87.50%> (ø)
internal/rules/mechanisms/prototype_repository.go	77.00% <88.88%> (ø)
internal/rules/mechanisms/unifiers/jwt_unifier.go	93.57% <90.90%> (ø)
...chanisms/contextualizers/generic_contextualizer.go	85.89% <100.00%> (ø)
internal/rules/mechanisms/factory.go	100.00% <100.00%> (ø)
internal/rules/mechanisms/unifiers/noop_unifier.go	100.00% <100.00%> (ø)
internal/rules/rule_factory.go	99.51% <100.00%> (ø)
... and 2 more

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