chore: Flatten validating webhook packages

[skhalash]

Description

Changes proposed in this pull request (what was done and why):

• Merge webhook handlers with validators, replace dependency injection with helper function calls
• Rewrite the unit tests to always call Handle and inspect the admission response
• Rewrite http.StatusBadRequest when admission request is denied

Changes refer to particular issues, PRs or documents:

Traceability

• The PR is linked to a GitHub issue.
• The follow-up issues (if any) are linked in the Related Issues section.
• If the change is user-facing, the documentation has been adjusted.
• If a CRD is changed, the corresponding Busola ConfigMap has been adjusted.
• The feature is unit-tested.
• The feature is e2e-tested.