fix: Cover additional Secretref validation scenarios

[TeodorSAP]

Description

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

• Set name, namespace, and key fields of the SecretKeyRef type as required
• Move secretref validation logic from API to secretref validator
• This solved the "Secret deleted after ConfigurationGenerated set to True" scenario, as well
• Add a new error message for missing fields in secret references
• Rename ReferencedSecretMissing to ReferencedSecretInvalid to also include the newly added error

Changes refer to particular issues, PRs or documents:

• Resolves secretRef without key does not fail the config generation #1506

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.