Use configmap for the VaultConnection

**Is your feature request related to a problem? Please describe.**
Give the choice to use configmap to share vault caCert.
The idea is to use [trust-manager](https://github.com/cert-manager/trust-manager/) to inject the caCert on the desired namespace. (like the kubeRootCa.crt provided by kubernetes)

**Describe the solution you'd like**
Add a field `caCertConfigMapRef`

**Describe alternatives you've considered**
Another solution is to have a webhook managed by the operator to inject the caCert directly on the vaultConnection.

Start with the VaultConnection API definition and trace how its caCert is currently supplied. Define how caCertConfigMapRef should identify the ConfigMap and how the referenced certificate is consumed; done means VaultConnection can use a trust-manager-populated ConfigMap in the desired namespace.

Written by the indexing model from the issue text.