**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).


"""
    Loads files from Google Cloud Storage into BigQuery.
    The schema to be used for the BigQuery table may be specified in one of
    two ways. You may either directly pass the schema fields in, or you may
    point the operator to a Google Cloud Storage object name. The object in
    Google Cloud Storage must be a JSON file with the schema fields in it.
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:GCSToBigQueryOperator`
    :param bucket: The bucket to load from. (templated)
    :param source_objects: String or List of Google Cloud Storage URIs to load from. (templated)
        If source_format is 'DATASTORE_BACKUP', the list must only contain a single URI.
    :param destination_project_dataset_table: The dotted
        ``(<project>.|<project>:)<dataset>.<table>`` BigQuery table to load data into.
        If ``<project>`` is not included, project will be the project defined in
        the connection json. (templated)
    :param schema_fields: If set, the schema field list as defined here:
        https://cloud.google.com/bigquery/docs/reference/v2/jobs#configuration.load
        Should not be set when source_format is 'DATASTORE_BACKUP'.
        Parameter must be defined if 'schema_object' is null and autodetect is False.
    :param schema_object: If set, a GCS object path pointing to a .json file that
        contains the schema for the table. (templated)
        Parameter must be defined if 'schema_fields' is null and autodetect is False.
    :param schema_object_bucket: [Optional] If set, the GCS bucket where the schema object
        template is stored. (templated) (Default: the value of ``bucket``)
    :param source_format: File format to export.
    :param compression: [Optional] The compression type of the data source.
        Possible values include GZIP and NONE.