Constraint get list

[lsviben]

What this PR does / why we need it:
Adds endpoints for LIST and GET for constraints. Also adds the constraint CRD and API objects.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #5899

Does this PR introduce a user-facing change?:

constraint get and list endpoints for V2:
GET /api/v2/projects/{project_id}/kubernetes/clusters/{cluster_id}/constraints/{constraint_name}
GET /api/v2/projects/{project_id}/kubernetes/clusters/{cluster_id}/constraints

[zreigz]

/lgtm
/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 6286a9e862e3eb80272eb7d05e107ca079e744a4

[lsviben]

/retest

[lsviben]

/retest

[moadqassem]

/approve
/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 3a303873dc95959ed12fc3098f2ef7e16c261d25

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lsviben, moadqassem, zreigz

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Needs approval from an approver in each of these files:

• OWNERS [lsviben,moadqassem,zreigz]
• cmd/kubermatic-api/OWNERS [lsviben,zreigz]
• pkg/api/OWNERS [lsviben,zreigz]
• pkg/crd/OWNERS [moadqassem]
• pkg/handler/OWNERS [lsviben,zreigz]
• pkg/provider/kubernetes/OWNERS [lsviben,zreigz]

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