[ENH] Add environment variable for allowed origins for CORS

[alyssadai]

Closes #150

Changes proposed in this pull request:

• New environment variable NB_API_ALLOWED_ORIGINS replaces hard-coded wildcard allowing all origins for CORS
  • default value is an empty string
• Add utility function to parse origins into a list
• Raise warning if no allowed origins are specified
• Tests added that specified origins are parsed correctly + warning raised appropriately

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[coveralls]

Pull Request Test Coverage Report for Build 6039803111

• 30 of 30 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 6016853072:	0.0%
Covered Lines:	368
Relevant Lines:	368

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💛 - Coveralls

[surchs]

Thanks for this PR @alyssadai. Everything functional is looking good! I still requested changes to make sure we communicate our expectations for CORS clearly to the user - I left some minor comments on where I think we should clarify things.

Take a look and let me know what you think. Then I think this can go

[surchs]

looks good @alyssadai, 🧑‍🍳