Implemented the BigQuery handler for Enrichment transform.

To use this handler you need either of the following combinations:

1. table_name, row_restriction_template, fields
2. table_name, row_restriction_template, condition_value_fn
3. query_fn

In addition to this, batch size can be specified with min_batch_size and max_batch_size. These are passed to the BatchElements transform just before making calls to BigQuery. This helps reducing the cost on the BigQuery side.

By default, the handler pulls all columns from the BigQuery table. To override this, use the column_name parameter to specify a list of column names to fetch.

The user interface here is pretty similar to TableReadOptions.

Example Usage:

handler = BigQueryEnrichmentHandler(
        project=PROJECT_ID,
        row_restriction_template="id='{}'",
        table_name=TABLE_NAME,
        fields=['id'],
        min_batch_size=2,
        max_batch_size=100,
    )

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