feat: add generic auth (#39)

* feat: add generic auth

Adds the ability to authenticate against any normal Vault endpoint by added the `authPayload` input.

When an unrecognized method is provided, the action will attempt to hit `v1/auth/<method>/login` with the provided `authPayload and parse out the token in the response

action.yml

@@ -33,6 +33,9 @@ inputs:
   githubToken:
     description: 'The Github Token to be used to authenticate with Vault'
     required: false
+  authPayload:
+    description: 'The JSON payload to be sent to Vault when using a custom authentication method.'
+    required: false
   extraHeaders:
     description: 'A string of newline separated extra headers to include on every request.'
     required: false