feat: add external-auth cli (#10052)

* feat: add `external-auth` cli

* Add subcommands

* Improve descriptions

* Add external-auth subcommand

* Fix docs

* Fix gen

* Fix comment

* Fix golden file

cli/testdata/coder_external-auth_access-token_--help.golden

@@ -0,0 +1,28 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder external-auth access-token [flags] <provider>
+
+  Print auth for an external provider
+
+  Print an access-token for an external auth provider. The access-token will be
+  validated and sent to stdout with exit code 0. If a valid access-token cannot
+  be obtained, the URL to authenticate will be sent to stdout with exit code 1
+    - Ensure that the user is authenticated with GitHub before cloning.:
+  
+       $ #!/usr/bin/env sh
+  
+  OUTPUT=$(coder external-auth access-token github)
+  if [ $? -eq 0 ]; then
+    echo "Authenticated with GitHub"
+  else
+    echo "Please authenticate with GitHub:"
+    echo $OUTPUT
+  fi
+
+OPTIONS:
+      --s bool
+          Do not print the URL or access token.
+
+———
+Run `coder --help` for a list of global options.