Add converters for HashiCorp Vault Plugin VaultCredential types:

• VaultAppRoleCredential
• VaultGitHubTokenCredential
• VaultTokenCredential

The following types are explicitly not supported:

• VaultTokenFileCredential - this credential types does not contain any secrets, only a reference to a file path where the secret is located.
• VaultKubernetesCredential - no secret data, just references to the role.
• VaultGCPCredential - no secret data, just reference to role and audience