[Feature Request] set user SQL/cassandra from env

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Is your feature request related to a problem? Please describe.

If we use a tool like bank-vault secrets webhook with dynamic engine vault, we need to get password AND user from an env var

That's why it will be cool to have user in env var :)

Describe the solution you'd like

See the pull request which will be associated,
I don't know if it is the best way to do it, but that's for the idea :)

Additional context

See:

• https://bank-vaults.dev/docs/mutating-webhook/
• https://developer.hashicorp.com/vault/docs/secrets/databases
• https://bank-vaults.dev/docs/mutating-webhook/deploy/#daemon-mode

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@robholland , @prathyushpv PR #517