[ENH] Add endpoint returning queryable attributes + refine existing endpoint for attribute instances

[alyssadai]

Closes #176
Closes #186

Changes proposed in this pull request:

• New router created for paths under the /attributes prefix
  • Existing endpoint for terms discovery: also moved under this prefix, i.e. /attributes/{data_element_URI}
• New endpoint created that returns all subclasses of nb:ControlledTerm
• Query string for existing terms discovery endpoint updated to return URIs for only instances of controlled term subclasses
• Helper function added to abbreviate namespaces in term URLs from the graph (e.g., "http://neurobagel.org/vocab/Diagnosis" -> "nb:Diagnosis")

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 6389516984

• 70 of 70 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 99.786%

---

Totals
Change from base Build 6343634889:	0.03%
Covered Lines:	467
Relevant Lines:	468

---

💛 - Coveralls

[rmanaem]

I left a comment regarding the new function in crud.py other than that it's good to go. 🧑‍🍳