Equivalent to withNumFileShards() in Python as it is in Java

We would like to have an equivalent of withNumFileShards() in Python as well

.apply("WriteInBigQueryLoad", BigQueryIO.writeTableRows().to(tableLowUrgency)
.withSchema(schema)
.withMethod(Method.FILE_LOADS)
.withTriggeringFrequency(Duration.standardMinutes(2))
.withNumFileShards(1)
.withCreateDisposition(BigQueryIO.Write.CreateDisposition.CREATE_IF_NEEDED)
.withWriteDisposition(BigQueryIO.Write.WriteDisposition.WRITE_APPEND))

transformed | 'Write' \>\> beam.io.WriteToBigQuery(
known_args.target_table,
schema=schema,
create_disposition=beam.io.BigQueryDisposition.CREATE_IF_NEEDED,
write_disposition=beam.io.BigQueryDisposition.WRITE_APPEND,
method=beam.io.WriteToBigQuery.Method.FILE_LOADS,
triggering_frequency=5
)

Imported from Jira [BEAM-8190](https://issues.apache.org/jira/browse/BEAM-8190). Original Jira may contain additional context.
Reported by: sdominique.

Start by locating Python's beam.io.WriteToBigQuery entry point and the Java BigQueryIO.writeTableRows implementation of withNumFileShards(). Compare their existing file-load options and related tests. Done means the Python API exposes equivalent file-shard behavior and its coverage verifies the requested configuration.

Written by the indexing model from the issue text.