Feature request: Add option to authenticate with AppRole

[andrewsav-bt]

Feature request:

Is this a front-end feature or backend? (or both?)
Mostly back-end with trivial front-end support.

Description of feature:
We currently have the following way to authenticate:

• Token
• Userpass
• LDAP
• Github
• OKTA

Can we add AppRole to this list?
The use case would be testing access to vault under an AppRole identity.

Are you able to submit a pull request for this feature?
Unlikely.

[Caiyeon]

This seems like a useful way to debug/test approle identities. However, I cannot prioritize it over other more pressing features & bug fixes. Pull requests are welcomed. Since this feature involves the backend and is easily testable, a proper pull request needs integration tests.