hwclt: Add cache for requests

[sergio-costas]

This PR adds a cache to preserve the latest state, and defines the cases where the cache has to be invalidated and the Certification Status must be requested again.

By default, the cache is stored at $SNAP_DATA, since the idea is to go full-snap.

• Unit tests
• Manual testing steps:

Built it and tested both "as-is" and with "sudo", both without parameters and with a wrong "--server" parameter. The first one returned, as expected, an error because it can't collect the data. The second succeeds. The third fails, as expected, with an error because the server doesn't return the expected data format.

Now it always returns a JSON through STDOUT, but if there is an error, it will also return it through STDERR as before.

Checklist

Still require more unitary tests.

• I have followed the contribution guidelines.
• I have signed the Canonical CLA.
• I have added necessary tests.
• I have added or updated any relevant documentation (if needed).
• I have tested the changes.

Additional Notes

[sergio-costas]

@pedro-avalos This is a proposal for adding the cache fields. What do you think?

[codecov]

Codecov Report

❌ Patch coverage is 73.52941% with 108 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.68%. Comparing base (aaab7e7) to head (513569a).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
- Coverage   89.17%   85.68%   -3.50%     
==========================================
  Files          29       29              
  Lines        1423     1781     +358     
  Branches        5        0       -5     
==========================================
+ Hits         1269     1526     +257     
- Misses        154      255     +101     

Flag	Coverage Δ		*Carryforward flag
charm	100.00% <ø> (ø)		Carriedforward from 72c8247
client	80.62% <ø> (+0.37%)	⬆️	Carriedforward from 72c8247
server	96.97% <ø> (ø)		Carriedforward from 72c8247

*This pull request uses carry forward flags. Click here to find out more.

Components	Coverage Δ
Server	96.85% <ø> (ø)
Charm	100.00% <ø> (+5.33%)	⬆️
Client	77.87% <73.52%> (-2.38%)	⬇️

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[pedro-avalos]

Thank you once again for the contribution and great ideas you've brought to Hardware API. I really appreciate the thought you've put behind your spec and willingness to contribute. I've laid out some feedback to make these changes in what I think could be a more structured form.

[sergio-costas]

@pedro-avalos Completed a proposal for the cache. Can you check it when you have some spare time just to ensure that is what you want, please?