[feature request] Allow ObjectStore config to use endpointCA from ConfigMap in addition to Secret

Currently, the ObjectStore config [only allows specifying the CA certificates from a secret](https://cloudnative-pg.io/plugin-barman-cloud/docs/object_stores/#using-object-storage-with-a-private-ca). We are using trust manager to distribute the certificates as config maps. The [`BarmanObjectStoreConfiguration`](https://pkg.go.dev/github.com/cloudnative-pg/barman-cloud/pkg/api#BarmanObjectStoreConfiguration) however only allows secrets to be specified.

e.g.:
```yaml
apiVersion: barmancloud.cnpg.io/v1
kind: ObjectStore
metadata:
name: my-obj-store
spec:
configuration:
endpointURL: https://
endpointCA:
configMapRef:
name: my-ca-configmap
key: ca.crt
```

The request names BarmanObjectStoreConfiguration and the ObjectStore configuration documentation; start by locating that API definition and the endpointCA handling. Support configMapRef alongside the existing secret reference, matching the YAML example, and verify the resulting ObjectStore configuration and validation behavior.

Written by the indexing model from the issue text.