Add support for a HashiCorp Vault getkey script

[apg]

Hi!

Was reading some stuff about Supabase Vault and wondered how to use HashiCorp Vault for the key, so decided to take a crack at something basic, inspired by the other getkey scripts.

Like the other getkey scripts, it hopes to find some environment variables, pertaining to Vault, and of course, uses env vars for the regular configuration.