build: add generated-files bot config

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Source-Link: https://github.com/googleapis/synthtool/commit/e6f3d54be015a394b6ab5a25903ec09062a2b424 Source-Link: https://github.com/googleapis/synthtool/commit/04573fd73f56791c659832aa84d35a4ec860d6f7

[codecov[bot]]

Codecov Report

Merging #242 (1a2e7a5) into master (81d8032) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #242   +/-   ##
=======================================
  Coverage   79.64%   79.64%           
=======================================
  Files           3        3           
  Lines         113      113           
  Branches       13       14    +1     
=======================================
  Hits           90       90           
  Misses         23       23           

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