[ENH] Add session ID as a new category for annotation

[surchs]

• Closes can annotate "Session ID" as phenotypic session (but can be empty if only for the purpose of listing the imaging session) #663

Changes proposed in this pull request:

• add Session ID as a category
• add new color for session id
• make sure session ID does not count as an annotation and does not get its own annotation workflow (like Subject ID)
• add a simple e2e test to make sure of this

Note: this should probably have more extensive tests, I went for a basic test because we may soon refactor the app. But could add appropriate tests as well.

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.

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[rmanaem]

Everything looks good 🧑‍🍳
It's a good idea to update the test containing the output validation using schema to ensure the output is valid after the session ID is annotated, wdyt?

[surchs]

good idea to update the test containing the output validation using schema to ensure the output is valid after the session ID is annotated

Yeah, I was thinking about that. The main thing is that our current example files don't have a session_id column and I didn't want to change the example / introduce a new one. I could just have it label an incorrect column as session_id and see what happens. Or do you have a better idea?

[rmanaem]

We could make an issue and address it later. The issue could include more than just updating the test e.g., adding a new example(s) that cover all columns and variables.