Github Auth method support

[jgoldverg]

Is your feature request related to a problem? Please describe.
Currently Vault supports Github Personal AccessTokens as an Auth method is there any plans to include this auth method into spring cloud vault and spring cloud vault config?

For a trivial solution I would expect I would expect that the property spring.cloud.vault.authentication would allow the field GITHUB where the spring.cloud.vault.token would contain the PAT. From the Vault documentation it seems like there is a separate API call for the Github Auth method which should make it fairly simple to include in a future version.

[mp911de]

There's already a PR at spring-projects/spring-vault#853 so we will merge that one to create pre-requisites.