Add tedge config agent.state.path setting

[didier-wenzek]

Proposed changes

• A new tedge-config setting has been added: agent.state.path
• This path is used to store the tedge-agent state across restarts.
• If for some reasons this directory cannot be used (doesn't exist or is not writable),
  then the tedge-agent use /etc/tedge/.agent as a fallback.
• The default value for agent.state.path is /data/tedge/agent.

agent.state.path
   The directory where the tedge-agent persists its state across restarts. 
   Note: If the given directory doesn't exists, `/etc/tedge/.agent` is used as a fallback irrespective of the current setting.
   Example: /data/tedge/agent

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

#2488

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

Merging #2492 (7747a16) into main (1c9e284) will increase coverage by 0.0%.
The diff coverage is 82.5%.

Additional details and impacted files

Files	Coverage Δ
...ates/core/tedge_agent/src/restart_manager/actor.rs	39.5% <100.0%> (+1.0%)	⬆️
...ates/core/tedge_agent/src/restart_manager/tests.rs	94.2% <100.0%> (+<0.1%)	⬆️
...tes/core/tedge_agent/src/software_manager/actor.rs	63.2% <100.0%> (-0.2%)	⬇️
...tes/core/tedge_agent/src/software_manager/tests.rs	93.7% <100.0%> (+<0.1%)	⬆️
.../tedge_config/src/tedge_config_cli/tedge_config.rs	80.8% <90.9%> (+0.1%)	⬆️
...tes/core/tedge_agent/src/state_repository/state.rs	91.9% <96.4%> (+1.2%)	⬆️
...tes/core/tedge_agent/src/restart_manager/config.rs	0.0% <0.0%> (-45.9%)	⬇️
...es/core/tedge_agent/src/software_manager/config.rs	0.0% <0.0%> (-34.6%)	⬇️

... and 2 files with indirect coverage changes

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
388	0	3	388	100	55m2.182999999s

[jarhodes314]

It's a bit surprising to see that this relies on the unhappy path to store in {temp_dir}/.agent

[didier-wenzek]

Yeah, it would be good to test the happy path too. Will do.

[didier-wenzek]

Done

[jarhodes314]

Looks good. I've left a couple of minor comments