Add built in Hashicorp vault resolver

[maxnorth]

The vault CLI is huge, and what's needed to retrieve secrets is just a simple HTTP request. It's not great having to add that CLI to a Docker image in order to use nv in containerized apps. Given vault's popularity, this seems like a good candidate to add as a built-in resolver.

Some research is needed of the common authentication methods supported by vault, to better understand which ones are best to start with and to anticipate how complicated supporting vault natively could become.