feat: auth mechanism for sass site to FC communication

[tanmoysrt]

Check the press/saas/api/readme.md of pull request for details.

Additional -

• Add these internal site config keys
  • fc_communication_secret ✅
  • subscription ✅

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 2.85714% with 34 lines in your changes missing coverage. Please review.

Project coverage is 37.50%. Comparing base (a995116) to head (cd9125a). Report is 51 commits behind head on master.

Files	Patch %	Lines
press/saas/api/__init__.py	0.00%	22 Missing :warning:
press/press/doctype/site/site.py	10.00%	9 Missing :warning:
press/utils/__init__.py	0.00%	2 Missing :warning:
press/saas/doctype/product_trial/product_trial.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2077 +/- ## ========================================== + Coverage 36.85% 37.50% +0.65% ========================================== Files 344 345 +1 Lines 28629 28906 +277 ========================================== + Hits 10551 10842 +291 + Misses 18078 18064 -14 ```

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