Support multiple modes of operation for vulnerability policies

[sahibamittal]

Description

At the moment, vulnerability policies are "enforced" as soon as they are created in the system.

There may be situations however, in which policies are supposed to be deployed, but not enforced. Add a mandatory field 'mode of operation' to define the same :

1. Disabled: Don't do anything with it, don't even consider it for evaluation.
2. Log: Evaluate policy, but don't take action, and log the outcome instead.
3. Apply: Apply policy analysis, perform db transactions and send notifications.

Addressed Issue

DependencyTrack/hyades#957

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 updated the migration changelog accordingly
• 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
✅ -0.03% (target: -1.00%)	✅ 85.00% (target: 70.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (4a0987b)	19568	15947	81.50%
Head commit (f2042f5)	19593 (+25)	15962 (+15)	81.47% (-0.03%)

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 (#669)	20	17	85.00%

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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[nscuro]

@sahibamittal There are still a few TODOs in here, should we convert this PR to Draft for now?