Add API and caching

[benoit74]

Changes:

• retrieve page information from Deki API, using an authentication token retrieved from home page
  • from my experience, the token is valid for multiple days, no worries to have here
• cache page and API responses on the file system so that one can re-use same JSON file over and over without having to worry

Nota: this is not yet used to populate anything in the ZIM, only "utility" code for now

To be reviewed after https://github.com/openzim/libretexts/pull/3 is merged

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 17.85714% with 92 lines in your changes missing coverage. Please review.

Project coverage is 44.94%. Comparing base (8efbbe5) to head (40cb342). Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/libretexts2zim/client.py	19.76%	69 Missing :warning:
scraper/src/libretexts2zim/entrypoint.py	16.66%	10 Missing :warning:
scraper/src/libretexts2zim/processor.py	11.11%	8 Missing :warning:
scraper/src/libretexts2zim/__main__.py	0.00%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4 +/- ## =========================================== - Coverage 56.72% 44.94% -11.79% =========================================== Files 7 7 Lines 238 336 +98 Branches 19 32 +13 =========================================== + Hits 135 151 +16 - Misses 102 184 +82 Partials 1 1 ```

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[benoit74]

I find we're duplicating the cache reading/writing bits ; maybe room for sharing this

This is probably because some of the code was buggy ^^ I fixed this. There is indeed only small variations ... but pretty important ones.

[benoit74]

I had to rebase to fix conflicts. Please review only two last commits which contains "real" modifications.

Nota: Iast commit is about making codecov informational. I find it's mostly mandatory since in scrapers like this one, most code is covered by integration tests, but these are not reported in codecov ...