Add cacheable variables support

[adammcdonagh]

This pull request adds support for cacheable variables. It includes the following changes:

• Added new cacheable variables

• Updated dependencies to include "omegaconf >= 2.3.0"

• Added a new JSON schema for a dummy source protocol

• Added a new exception class for caching plugin errors

• Updated VS Code settings to include write permissions for specific repositories in Codespaces

• Added new test cases for cacheable variables and caching plugins

• Added utility functions for updating the cache with new values

• Added a new JSON schema for cacheable variables

• Added new test cases for validating transfer JSON with dummy source and cacheable variables

These changes enable the caching of specific variables and provide flexibility for using different caching plugins.

[codecov]

Codecov Report

Attention: Patch coverage is 97.56098% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.49%. Comparing base (d82447d) to head (2011469).

Files	Patch %	Lines
...c/opentaskpy/config/variablecaching/cache_utils.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   88.34%   88.49%   +0.14%     
==========================================
  Files          18       20       +2     
  Lines        2257     2294      +37     
==========================================
+ Hits         1994     2030      +36     
- Misses        263      264       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.