Support Database Credential Types other than Password

[TimoVink]

Apologies if I missed something, but as far as I can tell VaultSharp only supports database secrets if the roles use the password credential_type.

The return type of IDatabaseSecretsEngine.GetCredentialsAsync is Secret<UsernamePasswordCredentials>, implicitly assuming the response from Vault will contain the username and password fields, but if, for example, you have configured the role with the rsa_private_key credential type, the response from Vault would (unsurprisingly) return a rsa_private_key field instead of password.

Link to the Vault API Docs that support this feature
https://developer.hashicorp.com/vault/docs/secrets/databases#credential-types
https://developer.hashicorp.com/vault/api-docs/secret/databases#credential_type

Further Context
Snowflake is deprecating the use of username/password authentication for service accounts. Forcing us to switch to other credential types.

[TimoVink]

I can probably make time to put in a PR for adding support for reading database secrets with other credential types, but would need some guidance from the maintainers on how they would like that implemented.

Just add a parallel GetPrivateKeyCredentialsAsync method on IDatabaseSecretsEngine? Breaking-ish change to the return type of the existing GetCredentialsAsync to something that can represent multiple types of credentials? Some other way?

[jhfeng]

we need support rsa key type asap, because Snowflake is deprecating username/password in few months.