feat(Bigquery): Append column samples

[yash30201]

Wrote and tested two samples -

• Add a column using a load job
• Add a column using a query job

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

• bigquery/api/src/add_column_load_append.php:32, tag bigquery_add_column_load_append
• bigquery/api/src/add_column_query_append.php:32, tag bigquery_add_column_query_append

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