refactor!: Subject has been made immutable

[dadrus]

Related issue(s)

addresses #1317

Prepares the code to support pre-authentication stages

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 introduces a breaking change and makes the Subject object immutable. That means it is not used to store results of any pipeline step any more. A new Outputs object has been introduced for that purpose instead.

Old Behavior

# rule
id: whatever
match:
  # match expression
execute:
  - authenticator: some_auth
  - contextualizer: some_data_fetcher # stored the results in Subject.Attribute.some_data_fetcher in the past
  - finalizer: create_header
    config:
      header:
         # access to the results from the "some_data_fetcher" step
         X-Some-Header: {{ .Subject.Attribute.some_data_fetcher | quote }}

New Behavior

# rule
id: whatever
match:
  # match expression
execute:
  - authenticator: some_auth
  - contextualizer: some_data_fetcher # stores now the results in Outputs.some_data_fetcher
  - finalizer: create_header
    config:
      header:
         # access to the results from the "some_data_fetcher" step
         X-Some-Header: {{ .Outputs.some_data_fetcher | quote }}

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 89.63%. Comparing base (55fb491) to head (e265c85).
Report is 2 commits behind head on main.

Files	Patch %	Lines
...nal/handler/envoyextauth/grpcv3/request_context.go	0.00%	4 Missing ⚠️
internal/handler/requestcontext/request_context.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1487   +/-   ##
=======================================
  Coverage   89.62%   89.63%           
=======================================
  Files         270      271    +1     
  Lines        9051     9075   +24     
=======================================
+ Hits         8112     8134   +22     
- Misses        694      699    +5     
+ Partials      245      242    -3     

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