Bigquery connector should not enable schema autodetection without a user explicitly instructing to do so.

Currently BQ_FILE_LOADS insertion method enables schema autodetection: https://github.com/apache/beam/blob/6567f1687d53e491b337ba94f521fa2e4af35e46/sdks/python/apache_beam/io/gcp/bigquery_tools.py#L340

 It may be more user-friendly allow users to opt-in for schema autodetection in their pipelines across all use-cases for BQ connector. Schema autodetection is an approximation, and does not always work.

For example, schema autodetection cannot infer whether a string data is binary bytes or textual string, and will always prefer the latter. If schema autodetection is enabled by default, users who need to write 'bytes' data will always have to specify a schema, even when writing to a table that was already created and has the schema. Otherwise autodetected schema will try to write 'string' entry into a 'bytes' field and the write will fail.

Related discussion: https://lists.apache.org/thread.html/1f9d9cb1bbbfca87d74e62ba8e58a15059ed6c20ab419002fcd3f8df@%3Cdev.beam.apache.org%3E