Add CLI arg to set dynamic config path

[Dean-Coakley]

Proposed changes

Fixes: #337

• Adds --dynamic-config-path cli arg which allows you to set a custom path to dynamic config file
• Update docs
• Simplify and extend config unit tests
• Fix typos

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (5d2118b) 66.25% compared to head (7dec14c) 66.26%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #490   +/-   ##
=======================================
  Coverage   66.25%   66.26%           
=======================================
  Files         118      118           
  Lines       13454    13457    +3     
=======================================
+ Hits         8914     8917    +3     
+ Misses       3945     3943    -2     
- Partials      595      597    +2     

Files	Coverage Δ
src/core/config/types.go	0.00% <ø> (ø)
...or/github.com/nginx/agent/v2/src/plugins/events.go	84.17% <100.00%> (ø)
src/core/config/defaults.go	13.63% <50.00%> (+13.63%)	⬆️
src/core/config/config.go	70.34% <33.33%> (+1.07%)	⬆️

... and 1 file with indirect coverage changes

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