fix(document): serialize @cache directives

[mogery]

Summary:
@cache directives are not serialized when calling config.to_sdl(). This PR adds logic to into_document.rs to fix this.

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eac8120) 88.39% compared to head (f3c40ab) 88.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1058      +/-   ##
==========================================
- Coverage   88.39%   88.38%   -0.01%     
==========================================
  Files          99       99              
  Lines       10721    10712       -9     
==========================================
- Hits         9477     9468       -9     
  Misses       1244     1244              

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[mogery]

This is all ready now. Should be a quick merge.

[mogery]

Pushed a new commit that fixes the identity fail of type-level cache directives, and adds a test for it. Should be good to go 👍🏻

[meskill]

Great work, thanks for the fix!