Adds config to get creds from vault

[Sahil-STFC]

closes: #47

[Sahil-STFC]

@dmbrownless @JonFijalkowski @ffionalys , I have named the secret to be merge-user-publisher-secrets let me know if you want a different name

[JonFijalkowski]

I don't think we're going with the name merge-user-publisher, since the publisher is technically purpose-agnostic - we can send other requests through it than merges.

I think the consensus would be to go with user-exchange-publisher, but Ffion wants to confirm all naming and things first. I'll put all of the decided names and things in https://github.com/isisbusapps/merge-user-tool/issues/384 once we have that.