Support pulling from private git repos

You can test this workflow with the procedure in
https://github.com/utoronto-2i2c/jupyterhub-deploy/#pulling-from-private-github-repos-with-nbgitpuller

You can test it with this link:
https://staging.utoronto.2i2c.cloud/hub/user-redirect/git-pull?repo=https%3A%2F%2Fgithub.com%2Futoronto-2i2c%2Fprivate-test-1&branch=main&urlpath=tree%2Fprivate-test-1%2FREADME.md

Fixes 2i2c-org#851

config/hubs/utoronto.cluster.yaml

@@ -57,6 +57,15 @@ hubs:
                 name: University of Toronto
                 url: https://www.utoronto.ca/
         singleuser: &staging_jhub_singleuser
+          extraFiles:
+            github-app-private-key.pem:
+              mountPath: /etc/github/github-app-private-key.pem
+            gitconfig:
+              mountPath: /etc/gitconfig
+              stringData: |
+                [credential "https://github.com"]
+                helper = !git-credential-github-app --app-key-file /etc/github/github-app-private-key.pem --app-id 93515
+                useHttpPath = true
           initContainers:
             # Need to explicitly fix ownership here, since Azure File doesn't do anonuid
             - name: volume-mount-ownership-fix

secrets/config/hubs/utoronto.cluster.yaml

@@ -2,6 +2,10 @@ hubs:
     - name: ENC[AES256_GCM,data:IAeq1UJtaQ==,iv:uw3u83ri3uxGI0kgRsg6F+eEFFf4p6OZXGJRicLMl00=,tag:2OZjavRba3MlN+0CJxVzaA==,type:str]
       config:
         jupyterhub:
+            singleuser:
+                extraFiles:
+                    github-app-private-key.pem:
+                        stringData: ENC[AES256_GCM,data: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,iv:vMvkAvAxetYx0JTzWkmlsicyGjgspA8eMeL0KbpV1xg=,tag:yUm7SZ9s6/zktLWBXyAiiw==,type:str]
             hub:
                 config:
                     AzureAdOAuthenticator:
@@ -10,6 +14,10 @@ hubs:
     - name: ENC[AES256_GCM,data:hIMyvQ==,iv:jSMZit0o6ai3yxJjRMe6rLRgGYfeqzWtS4ntBbGVst4=,tag:5qp5pzwfB7SLf+ItXekYdQ==,type:str]
       config:
         jupyterhub:
+            singleuser:
+                extraFiles:
+                    github-app-private-key.pem:
+                        stringData: ENC[AES256_GCM,data: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,iv:vMvkAvAxetYx0JTzWkmlsicyGjgspA8eMeL0KbpV1xg=,tag:yUm7SZ9s6/zktLWBXyAiiw==,type:str]
             hub:
                 config:
                     AzureAdOAuthenticator:
@@ -25,8 +33,8 @@ sops:
     azure_kv: []
     hc_vault: []
     age: []
-    lastmodified: "2021-12-13T09:04:55Z"
-    mac: ENC[AES256_GCM,data:0j8kcivCARJMtCaM4aGwKD4Nvbr8LzGU2+m1+XcTLcRDwlm2IyN+CLAugmX5NHyHk/21Vz743FCTEHWd4j62eAt4EkjVsRO5pST42rh89g38lC9/9gudu8Q3EbM1uN46vCUetqFW2YrqnGgjjwYWT7K/gfDiIWquSG29c6L5LYs=,iv:jyqLmr2l2iNp46UOpWBmeDmphvGYM0ZzrHqFgz3fOZ0=,tag:051lQDmd3JGx1oBWhz6tJA==,type:str]
+    lastmodified: "2021-12-13T15:37:12Z"
+    mac: ENC[AES256_GCM,data:JJD8IM0P+ifwU0tcqNT5Wtvs4140tsj9l9Vc/5AJpHjsTlP1QRw7Jh4tgVk6/Rw3jzSWmGhxp3GEtbV2pQ/WAOixKwC3wN9fWarDaguvYNS89xW0/fAHBlRwoBnga5KErUd5g/S7+Nv/gc1UObj22qUfnRJ83Tb2ekVuRjLJx2c=,iv:rU6MamjwmrJpBWhp1hljyX5iPZ5YA4X93vv9NYReEvU=,tag:vUIo3ApZxmRMP4fwJ76oJQ==,type:str]
     pgp: []
     unencrypted_suffix: _unencrypted
     version: 3.7.1