Support managing secrets

[benesch]

It'd be great if this provider supported a resource_secret that would allow Terraform to manage Supabase secrets—i.e., the functionality that's currently available via the CLI and API as:

• supabase secrets set https://supabase.com/docs/reference/cli/supabase-secrets-set
• POST v1/projects/<project>/secrets: https://api.supabase.com/api/v1#tag/secrets/POST/v1/projects/{ref}/secrets