feat: @validate directive for custom scalars

[mobley-trent]

Summary:
Adds the @validate directive for custom scalars. Currently, there is no mechanism to validate custom scalars as opposed to the other types in Tailcall.

Issue Reference(s):
/claim #1847
closes #1847

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

Attention: Patch coverage is 83.33333% with 2 lines in your changes are missing coverage. Please review.

Please upload report for BASE (main@756a56f). Learn more about missing BASE report.
Report is 44 commits behind head on main.

❗ Current head dcb32d9 differs from pull request most recent head a8ec8a1

Please upload reports for the commit a8ec8a1 to get more accurate results.

Files	Patch %	Lines
src/config/config.rs	60.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1889   +/-   ##
=======================================
  Coverage        ?   89.70%           
=======================================
  Files           ?      154           
  Lines           ?    15849           
  Branches        ?        0           
=======================================
  Hits            ?    14217           
  Misses          ?     1632           
  Partials        ?        0           

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