docs(samples): add load table and truncate data from gcs using avro file

[pmakani]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #567 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #567   +/-   ##
=========================================
  Coverage     81.49%   81.49%           
  Complexity     1226     1226           
=========================================
  Files            77       77           
  Lines          6220     6220           
  Branches        691      690    -1     
=========================================
  Hits           5069     5069           
  Misses          792      792           
  Partials        359      359           

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