docs(samples): add load table 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 #564 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #564      +/-   ##
============================================
+ Coverage     81.45%   81.49%   +0.04%     
- Complexity     1225     1226       +1     
============================================
  Files            77       77              
  Lines          6200     6220      +20     
  Branches        677      691      +14     
============================================
+ Hits           5050     5069      +19     
- Misses          791      792       +1     
  Partials        359      359              

Impacted Files	Coverage Δ	Complexity Δ
...e/cloud/bigquery/testing/RemoteBigQueryHelper.java	61.40% <0.00%> (-0.42%)	6.00% <0.00%> (ø%)
...ain/java/com/google/cloud/bigquery/Clustering.java	100.00% <0.00%> (ø)	5.00% <0.00%> (ø%)
...ain/java/com/google/cloud/bigquery/PolicyTags.java	100.00% <0.00%> (ø)	6.00% <0.00%> (ø%)
...java/com/google/cloud/bigquery/BigtableColumn.java	100.00% <0.00%> (ø)	5.00% <0.00%> (ø%)
...java/com/google/cloud/bigquery/TimelineSample.java	100.00% <0.00%> (ø)	5.00% <0.00%> (ø%)
...om/google/cloud/bigquery/ModelTableDefinition.java	100.00% <0.00%> (ø)	4.00% <0.00%> (ø%)
...om/google/cloud/bigquery/LoadJobConfiguration.java	93.20% <0.00%> (+0.02%)	56.00% <0.00%> (ø%)
...ogle/cloud/bigquery/WriteChannelConfiguration.java	86.81% <0.00%> (+0.06%)	46.00% <0.00%> (ø%)
...google/cloud/bigquery/StandardTableDefinition.java	93.33% <0.00%> (+0.09%)	16.00% <0.00%> (ø%)
...om/google/cloud/bigquery/BigtableColumnFamily.java	96.00% <0.00%> (+0.16%)	5.00% <0.00%> (ø%)
... and 8 more

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