add docs for gcp iap

Author: msukkari

docs/docs/configuration/auth/overview.mdx

@@ -80,6 +80,16 @@ Optional environment variables:
 - `AUTH_EE_GOOGLE_CLIENT_ID`
 - `AUTH_EE_GOOGLE_CLIENT_SECRET`
 
+### GCP IAP
+---
+
+Custom provider built to enable automatic Sourcebot account registration/login when using GCP IAP.
+
+**Required environment variables**
+- `AUTH_EE_GCP_IAP_ENABLED`
+- `AUTH_EE_GCP_IAP_AUDIENCE`
+    - This can be found by selecting the ⋮ icon next to the IAP-enabled backend service and pressing `Get JWT audience code`
+
 ### Okta
 ---
 

docs/docs/configuration/environment-variables.mdx

@@ -51,6 +51,8 @@ The following environment variables allow you to configure your Sourcebot deploy
 | `AUTH_EE_OKTA_CLIENT_ID` | `-` | <p>The client ID for Okta SSO authentication.</p> |
 | `AUTH_EE_OKTA_CLIENT_SECRET` | `-` | <p>The client secret for Okta SSO authentication.</p> |
 | `AUTH_EE_OKTA_ISSUER` | `-` | <p>The issuer URL for Okta SSO authentication.</p> |
+| `AUTH_EE_GCP_IAP_ENABLED` | `false` | <p>When enabled, allows Sourcebot to automatically register/login from a successful GCP IAP redirect</p> |
+| `AUTH_EE_GCP_IAP_AUDIENCE` | - | <p>The GCP IAP audience to use when verifying JWT tokens. Must be set to enable GCP IAP JIT provisioning</p> | 
 
 
 ### Review Agent Environment Variables