Add Hashicorp Vault service

What does this Merge Request do and why?

This MR adds the Hashicorp Vault vault Homebrew package to GDK's Brewfile. It allows a Vault service to be configured in gdk.yml using:

vault:
  enabled: true
  listen_address: 127.0.0.1 # optional

Vault can then be started with gdk start vault.

Second MR for gitlab#369351 (closed)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.