feat: Cluster level default settings for Hashicorp Vault

[sukratkashyap]

This PR enables global default Vault Role and Vault Mount Point setting for k8s authentication to Hashicorp Vault.

Reason:
When a user tries to deploy an app to multiple different cluster which uses the same vault. The External Secret yaml file has differences since, vaultRole and vaultMountPoint could be different for different clusters. This leads to multiple External Secret yaml file. One for each cluster. Whereas this could be specified during deployment of External Secret helm chart.

Since, a cluster will most probably will have one vaultMountPoint. It is better to have a way to deploy it at the config level. Leaving the user not to worry about role and mount point and focus on the values retrieved from the vault. Since, it is a default value. This will be backwards compatible with the previous method as it can be overwritten with the value in the External Secret manifest file.

[megakid]

Note that my new PR would be compatible with this change. #488

[Flydiverny]

LGTM