chore(java): enable generated-files-bot (#431)

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Synth log will be available here:
https://source.cloud.google.com/results/invocations/144f0dc1-363d-4512-9d09-acba1f17f7e5/targets

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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

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-asset.git",
-        "sha": "7631fa1faa48fbcbfaeb1e07f743955e096da64c"
+        "sha": "dacfdc7b2db6fc58500eb53a161bde791d84c84f"
       }
     },
     {
@@ -59,7 +59,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "f68649c5f26bcff6817c6d21e90dac0fc71fef8e"
+        "sha": "b19b401571e77192f8dd38eab5fb2300a0de9324"
       }
     }
   ],
@@ -125,6 +125,7 @@
     ".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",