Allow another process (eg Vault Agent)

[rdeanmcdonald]

Hello Lucretius! I have a Vault setup such that a Vault Agent process is already running next to my Vault Server nodes. This Vault Agent uses auto_auth with AWS, and writes a token to a sink file. I wanted to have your raft snapshot agent use that token, instead of authenticate via approle. I made a couple minor additions and figured I'd create a pull request in case it's worth adding to your project.

VSCode did some auto formatting which formatted a couple spots that I otherwise would not have touched. I can undo those if this is something you want to pull into your project