Add ability to override github app callback url

By default, the github app provider redirects back to the base url.
This change allows a user to override the GitHub App url using the config field `customCallbackUrl`.

It will look like this in the configuration:
```
auth:
  providers:
    github:
      development:
        clientId: SUPER ID
        clientSecret: SUPER SECRET
        customCallbackUrl: https://some-url.com/callback
```

Signed-off-by: Nicolas Arnold <nic@roadie.io>

.changeset/pretty-feet-explode.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Allow users to override callback url of Github provider

plugins/auth-backend/src/providers/github/provider.ts

@@ -223,6 +223,9 @@ export const createGithubProvider = (
       const enterpriseInstanceUrl = envConfig.getOptionalString(
         'enterpriseInstanceUrl',
       );
+      const customCallbackUrl = envConfig.getOptionalString(
+        'customCallbackUrl',
+      );
       const authorizationUrl = enterpriseInstanceUrl
         ? `${enterpriseInstanceUrl}/login/oauth/authorize`
         : undefined;
@@ -232,7 +235,8 @@ export const createGithubProvider = (
       const userProfileUrl = enterpriseInstanceUrl
         ? `${enterpriseInstanceUrl}/api/v3/user`
         : undefined;
-      const callbackUrl = `${globalConfig.baseUrl}/${providerId}/handler/frame`;
+      const callbackUrl = customCallbackUrl ||
+       `${globalConfig.baseUrl}/${providerId}/handler/frame`;
 
       const catalogIdentityClient = new CatalogIdentityClient({
         catalogApi,