Add REST endpoints for tag retrieval

[nscuro]

Description

Adds REST endpoints for tag retrieval:

Addressed Issue

Relates to #586
Closes #3886

Additional Details

Response of /api/v1/tag:

Response of /api/v1/tag/{name}/policy:

Response of /api/v1/tag/{name}/project:

Frontend PR: DependencyTrack/frontend#922

Checklist

• I have read and understand the contributing guidelines
• This PR fixes a defect, and I have provided tests to verify that the fix is effective
• This PR implements an enhancement, and I have provided tests to verify that it works as intended
• This PR introduces changes to the database model, and I have added corresponding update logic
• This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.02% (target: -1.00%)	✅ 79.84% (target: 70.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (e3217a3)	21794	16563	76.00%
Head commit (24e1773)	21904 (+110)	16651 (+88)	76.02% (+0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3881)	124	99	79.84%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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