Add REST endpoints for tag retrieval

[nscuro]

Description

Adds REST endpoints for tag retrieval:

Addressed Issue

Relates to https://github.com/DependencyTrack/dependency-track/issues/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: https://github.com/DependencyTrack/frontend/pull/922

Checklist

• [x] 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~
• [x] 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[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

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

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (e3217a373c110e817e4f4317af44c3c6840864ab) | 21794 | 16563 | 76.00% | | | Head commit (24e1773b01c5d11dfa5d72c0f3861df14bbb31f6) | 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: ` - `

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: `/ * 100%`

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