**Description**

We would like to be able to load data to **existing** BigQuery tables without having to specify schema_fields/schema_object in `GCSToBigQueryOperator` since table already exists.

**Use case / motivation**
<details>
  <summary>BigQuery load job usage details and problem explanation</summary>
  
We create BigQuery tables/datasets through CI process (terraform managed), with the help of Airflow we updating those tables with data.
To update tables with data we can use:
Airflow 2.0 operator: GCSToBigQueryOperator
Airflow 1.* operator (deprecated) GoogleCloudStorageToBigQueryOperator 
However those operator require to specify one of 3 things:
1. schema_fields - fields that define table schema
2. schema_object -  a GCS object path pointing to a .json file that contains the schema for the table
3. or autodetect=True
In other cases it will:
```
 raise ValueError('At least one of `schema_fields`, `schema_object`, '
                                 'or `autodetect**=True**` must be passed.')  
```
_Note: it does not actually says that `autodetect` must be `True` in exception - but according to code it must be specified as True, or schema should be used otherwise._

But we already have created table, and we can update it using 
`bq load` command. (which Airflow operators mentioned above are using internally)

When using `bq load` - you also have an option to specify **schema**. The schema can be a local JSON file, or it can be typed inline as part of the command. You can also use the `--autodetect` flag instead of supplying a schema definition.
https://cloud.google.com/bigquery/docs/loading-data-cloud-storage-csv#bq

When you specify `--autodetect` as True - BigQuery will try to give random names to your columns, e.g.: 'string_field_0', 'int_field_1' - and if you are trying to load into **existing** table - `bq load` will fail with error:
'Cannot add fields (field: string_field_0)'}.'
Same way Airflow operators like 'GCSToBigQueryOperator' will fail.


However **there is also an option NOT to specify** `--autodetect` or specify `--autodetect=False` and in this case `bq load` will load from CloudStorage to **existing** BQ table without problems.
</details>


Proposal/TL;DR: 
Add an option **not** to specify `--autodetect` or specify `--autodetect=False` when  write_disposition='WRITE_APPEND' is used in GCSToBigQueryOperator. This will allow an operator to update **existing** BigQuery table without having to specify schema within the operator itself (it will just be updating **existing** table with data).


self.schema_object_bucket = schema_object_bucket
        
        self.destination_project_dataset_table = destination_project_dataset_table
        self.schema_fields = schema_fields
        if source_format.upper() not in ALLOWED_FORMATS:
            raise ValueError(
                f"{source_format} is not a valid source format. "
                f"Please use one of the following types: {ALLOWED_FORMATS}."
            )
        else:
            self.source_format = source_format.upper()
        self.compression = compression
        self.create_disposition = create_disposition
        self.skip_leading_rows = skip_leading_rows
        self.write_disposition = write_disposition
        self.field_delimiter = field_delimiter
        self.max_bad_records = max_bad_records
        self.quote_character = quote_character
        self.ignore_unknown_values = ignore_unknown_values
        self.allow_quoted_newlines = allow_quoted_newlines
        self.allow_jagged_rows = allow_jagged_rows
        self.external_table = external_table
        self.encoding = encoding
        self.max_id_key = max_id_key
        self.gcp_conn_id = gcp_conn_id
        self.delegate_to = delegate_to
        self.schema_update_options = schema_update_options
        self.src_fmt_configs = src_fmt_configs
        self.time_partitioning = time_partitioning
        self.cluster_fields = cluster_fields