feat: add `.spec.replicaCluster.primary` field

[armru]

Closes #3942

[github-actions[bot]]

:exclamation: By default, the pull request is configured to backport to all release branches.

• To stop backporting this pr, remove the label: backport-requested :arrow_backward: or add the label 'do not backport'
• To stop backporting this pr to a certain release branch, remove the specific branch label: release-x.y

[armru]

/test limit=local

[github-actions[bot]]

@armru, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/8847312403

[armru]

/test limit=local

[github-actions[bot]]

@armru, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/8847457676

[armru]

/test limit=local

[github-actions[bot]]

@armru, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/8875148319

[litaocdl]

https://github.com/EnterpriseDB/cloudnative-pg/actions/runs/9250910553

[leonardoce]

E2e: https://github.com/EnterpriseDB/cloudnative-pg/actions/runs/9580425444

[leonardoce]

/ok-to-merge e2e failures are not related