Default vaultauthglobal in helm chart

**Is your feature request related to a problem? Please describe.**
Currently, there is no ability to manage a default VaultAuthGlobal configuration that can be used across all Kubernetes namespaces.

**Describe the solution you'd like**
As a consumer of this chart, it would be beneficial to manage a default VaultAuthGlobal configuration in a manner similar to defaultVaultConnection and defaultAuthMethod. This would allow consumers of the Vault Secrets Operator (VSO) to create custom VaultAuth configurations in their namespaces, only needing to specify their unique roles.

**Describe alternatives you've considered**
An alternative approach is to create and manage the VaultAuthGlobal configuration using a Terraform resource outside of the VSO Helm chart. Since the VaultAuthGlobal crd is scoped namespace it cannot be used across all namespace so this only helps if you have multiple custom VaultAuths in each namespace.

**Additional context**
NA