feat: sample - add col load append

[stephaniewang526]

Fixes b/147690070

[codecov]

Codecov Report

Merging #112 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #112      +/-   ##
============================================
+ Coverage     77.32%   77.38%   +0.05%     
- Complexity     1108     1110       +2     
============================================
  Files            73       73              
  Lines          5915     5916       +1     
  Branches        645      645              
============================================
+ Hits           4574     4578       +4     
+ Misses         1014     1013       -1     
+ Partials        327      325       -2

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/bigquery/spi/v2/HttpBigQueryRpc.java	5.2% <0%> (-0.02%)	2% <0%> (ø)
...google/cloud/bigquery/StandardTableDefinition.java	92.64% <0%> (+5.88%)	16% <0%> (+2%)	⬆️

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[kurtisvg]

Added comments on checked vs unchecked.