Add GitHub auth method.

[brendonjwong]

Implemented the GitHub auth's login method. This should resolve https://github.com/amperity/vault-clj/issues/79.

Validated with a local vault server and my personal Github token, using the Amperity organization. I figured it probably didn't make sense to have an integration test for this, but I'm open to ideas for how to durably test this.

Validation steps:

• In the terminal:

  
  ❯ vault auth enable -path=test-mount github
  Success! Enabled github auth method at: test-mount/

❯ vault write auth/test-mount/config organization=amperity Success! Data written to: auth/test-mount/config

❯ vault write auth/test-mount/map/teams/developers value=default Success! Data written to: auth/test-mount/map/teams/developers

- In the REPL:
```clojure
vault.repl=> (def client (http/http-client "http://127.0.0.1:8200"))
#'vault.repl/client

vault.repl=> (require '[vault.auth.github :as gh])
nil

vault.repl=> (gh/login (gh/with-mount client "test-mount") "<redacted>")
{:accessor "<redacted>",
 :client-token "<redacted>",
 :entity-id "<redacted>",
 :lease-duration 2764800,
 :metadata {:org "amperity", :username "brendonjwong"},
 :mfa-requirement nil,
 :num-uses 0,
 :orphan true,
 :policies ["default"],
 :renewable true,
 :token-policies ["default"],
 :token-type "service"}

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (vault-2.x-rewrite@841ced8). Click here to learn what that means. Patch has no changes to coverable lines.

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