Konnect entities: Implement CA Certificate refs for KongService and KongUpstream

[pmalek]

Problem statement

Service and Upstream types in Konnect APIs contain a ca_ceritificates field which is described as:

Array of CA Certificate object UUIDs that are used to build the trust store while verifying upstream server's TLS certificate. If set to null when Nginx default is respected. If default CA list in Nginx are not specified and TLS verification is enabled, then handshake with upstream server will always fail (because no CA are trusted).

Given that the operator works in Kubernetes environment, we'd probably want to implement that as a list of references to in cluster resources instead of a verbatim list of UUIDs.

Acceptance criteria

• As a user I can set the ca_certificates field on KongService and KongUpstream objects when working with Konnect entities.