Add ability to set custom config merge plan file path, config and vendor directories

[vjik]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌

[what-the-diff]

PR Summary

• Enhancement in Configuration Customization
  The team has made updates to allow for setting a custom config merge plan file path, and specifying config and vendor directories in the Application Runner. This allows a much deeper level of configuration customization.

• Test Addition for Configuration
  Tests have been added for the config merge plan file and config directory in the Application Runner Test file. This ensures the newly introduced feature works as expected.

• New Properties Introduced in Application Runner
  The Application Runner now has configDirectory and vendorDirectory. This gives additional control in specifying locations of config files which is helpful for maintaining organized codebases.

• New Files in the Custom Config Directory
  Files named test-merge-plan.php, .merge-plan.php, and params.php have been added to the custom-config directory in the test space. These new files will further enable testing and validation of the application running phase.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (924f7c9) to head (f5ef1e0).

Additional details and impacted files

@@             Coverage Diff             @@
##              master       #58   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        23        23           
===========================================
  Files              2         2           
  Lines             71        72    +1     
===========================================
+ Hits              71        72    +1     

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