feat(Prophet-coeffs): DS-2107 store prophet coeffs

[Eliel-Avilez]

Project Robyn

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue)

Type of change

• fix: Bug fix (non-breaking change which fixes an issue)
• feat: New feature (non-breaking change which adds functionality)
• docs: API change with a documentation update
• test: Additional test coverage
• reformat: Code cleanup, recoding or reformating code

How Has This Been Tested?

• Fork the repo and create your branch from master (latest dev version).
• If you've changed docuemntation, run devtools::document(). Should update .Rd files.
• Ensure all tests pass, run devtools::check(). Should have no notes, no warning, no errors.
• Make sure your code lints.

[GG-TechOpsSuper]

✅ Snyk checks have passed. No issues have been found so far.

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