BigQuery: refactor bigquery_load_* code samples to pass parameters to LoadJobConfig constructor

[emar-kar]

Is your feature request related to a problem? Please describe.
Related to #9596. Since query_parameters will be passed directly into the QueryJobConfig constructor I assume the same should be applied to the parameters in LoadJobConfig constructor as well. I consider that it will help to simplify and optimize the code of the request.

google-cloud-python/bigquery/docs/snippets.py

Lines 678 to 681 in 38d8f2b

	job_config = bigquery.LoadJobConfig()
	job_config.source_format = bigquery.SourceFormat.CSV
	job_config.skip_leading_rows = 1
	job_config.autodetect = True

google-cloud-python/bigquery/docs/snippets.py

Lines 746 to 753 in 38d8f2b

	job_config = bigquery.LoadJobConfig()
	job_config.schema = [
	bigquery.SchemaField("name", "STRING"),
	bigquery.SchemaField("post_abbr", "STRING"),
	]
	job_config.skip_leading_rows = 1
	# The source format defaults to CSV, so the line below is optional.
	job_config.source_format = bigquery.SourceFormat.CSV

Describe the solution you'd like
A clear and concise description of what you want to happen.

• Move any LoadJobConfig snippets into the samples folder
• Unify the LoadJobConfig constructor initialization same way as in 9495

e.g.,

job_config = bigquery.LoadJobConfig(
        schema=[
            bigquery.SchemaField("name", "STRING"),
            bigquery.SchemaField("post_abbr", "STRING"),
            bigquery.SchemaField("date", "DATE"),
        ],
        skip_leading_rows=1,
        time_partitioning=bigquery.TimePartitioning(
            type_=bigquery.TimePartitioningType.DAY,
            field="date",  # Name of the column to use for partitioning.
            expiration_ms=7776000000,  # 90 days.
        ),
    )

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
N/A
Additional context
Add any other context or screenshots about the feature request here.
N/A

[emar-kar]

@tswast, please comment