chore(java): enable generated-files-bot (#477)

This bot will comment on pull requests that modify templated files. For java, we configure to read from our 2 synth.metadata files: (primary templates and README)

Source-Author: Jeff Ching <chingor@google.com>
Source-Date: Fri Oct 30 14:22:02 2020 -0700
Source-Repo: googleapis/synthtool
Source-Sha: b19b401571e77192f8dd38eab5fb2300a0de9324
Source-Link: googleapis/synthtool@b19b401

.github/generated-files-bot.yml

@@ -0,0 +1,7 @@
+externalManifests:
+- type: json
+  file: 'synth.metadata'
+  jsonpath: '$.generatedFiles[*]'
+- type: json
+  file: '.github/readme/synth.metadata/synth.metadata'
+  jsonpath: '$.generatedFiles[*]'

synth.metadata

@@ -11,7 +11,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-bigquerydatatransfer.git",
-        "sha": "8023ba4af8b5f56a110242930d342ea5626f7662"
+        "sha": "06a6f2ca16810700cbc65f6d9bb9a370ea679f06"
       }
     },
     {
@@ -26,7 +26,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "5f6ef0ec5501d33c4667885b37a7685a30d41a76"
+        "sha": "b19b401571e77192f8dd38eab5fb2300a0de9324"
       }
     }
   ],
@@ -47,6 +47,7 @@
     ".github/ISSUE_TEMPLATE/feature_request.md",
     ".github/ISSUE_TEMPLATE/support_request.md",
     ".github/PULL_REQUEST_TEMPLATE.md",
+    ".github/generated-files-bot.yml",
     ".github/readme/synth.py",
     ".github/release-please.yml",
     ".github/snippet-bot.yml",