feat: Add is_producer_url_unstable to feed schema and expose in API (#1766)

[jcpitre]

Closes #1766

• Added is_producer_url_unstable in the DB schema.
• Modified the populate scripts to accept the new flag.
• Exposed the flag in the public API, in /v1/feed and /v1/search returns

The rule for updating the DB from the csv file is:

• An empty is_producer_url_unstable cell in the csv will be a no-change in the DB.
• An explicit TRUE or FALSE value in the csv will overwrite the current value.
• I believe it's the way it was done with is_official.

Note: This PR should be merged before this PR from mobility-database-catalogs: Add unstable_producer_url field
#1547

Testing tips:

This PR includes new unit tests.
The whole chain (mobility-database-catalogs -> mobility-feed-api) was tested somewhat, except the notification between the repos that is outside of scope.
The test was:

• In a branch in mobility-database-catalogs, add the is_producer_url_unstable flag:

  • "is_producer_url_unstable": "True",
    

  • to one json file, and set it to False in another one.
• Run the Export catalogs to CSV workflow.
  • Note that this workflow was changed so it can accept a different file name than source.csv. The file will be put in the placed in the mdb-csv (authentication required) bucket.
• Run the Mobility Catalog Update workflow in the current repo.
  • Note that the workflow was modified to be able to specify the filename that was used when creating the csv.

Tested these cases:

Catalog JSON value	Existing DB value	Resulting DB value	Note
True	any	true	explicit value always wins
False	any	false	explicit value always wins
absent / empty	null	null	new/unknown feed stays unknown
absent / empty	true	true	retained — empty does not clobber
absent / empty	false	false	retained — empty does not clobber

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• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
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