refactor(bigquery): update code samples to use strings for table and dataset IDs

[emar-kar]

Towards #8989
Closes #9499.

List of changes:

New samples:

1. client_query_add_column;
2. client_query_batch;
3. client_query_destination_table;
4. client_query_destination_table_cmek;
5. client_query_destination_table_legacy;
6. client_query_dry_run;
7. client_query_legacy_sql;
8. client_query_relax_column;
9. copy_table_cmek;
10. copy_table_multiple_source.

Comment lines in samples unified:

1. client_query;
2. copy_table;
3. load_table_dataframe.

DEPRECATED tests

1. test_create_routine;
2. test_update_routine;
3. test_get_routine;
4. test_delete_routine;
5. test_list_routines;
6. test_create_routine_ddl

All those tests were united in the test_routine_samples file!

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[tswast]

Let's call dest_table = client.get_table(random_table_id) and then assert dest_table.num_rows > 0 so that we know that the job did what we expected it to do.

[tswast]

Use random_table_id since we want to create a new table in the actual sample.

The table_id creates a table before the tests run.

google-cloud-python/bigquery/samples/tests/conftest.py

Line 76 in 9360057

table = client.create_table(table)

[tswast]

I got confused by this test.

I'd very much like to see a positive assertion that a non-zero number is present. not in could happen for many different reasons.

Suggested change

	assert "This query will process 0 bytes." not in out
	assert "This query will process" in out
	assert query_job.total_bytes_processed > 0