fix: Loading of structured configuration from env variables

[dadrus]

Related issue(s)

closes #769

Checklist

• I agree to follow this project's Code of Conduct.
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• I have referenced an issue describing the bug/feature request.
• I have added tests that prove the correctness of my implementation.

Description

Heimdall supports loading of the entire configuration from environment variables in addition to a config file. Merging is also supported as described in Configuration Introduction. However, if the relevant configuration part resides in an array, e.g.

rules:
  mechanisms:
    authenticators:
      - id: jwt
        type: jwt
        config:
           jwks_endpoint: http://whatever
           # further config properties   

configuring the structured parts, or overriding these, like here the properties under the config key, is not possible.

This PR fixes that and makes the expected configuration possible.

[codecov]

Codecov Report

Merging #768 (88ab42e) into main (2ad82a6) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #768   +/-   ##
=======================================
  Coverage   89.59%   89.59%           
=======================================
  Files         215      215           
  Lines        9223     9223           
=======================================
  Hits         8263     8263           
  Misses        767      767           
  Partials      193      193           

Files Changed	Coverage Δ
internal/config/parser/merge.go	90.90% <100.00%> (ø)

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