**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).


The default value is NONE.
        This setting is ignored for Google Cloud Bigtable,
        Google Cloud Datastore backups and Avro formats.
    :param create_disposition: The create disposition if the table doesn't exist.
    :param skip_leading_rows: The number of rows at the top of a CSV file that BigQuery
        will skip when loading the data.
        When autodetect is on, the behavior is the following:
        skip_leading_rows unspecified - Autodetect tries to detect headers in the first row.
        If they are not detected, the row is read as data. Otherwise, data is read starting
        from the second row.
        skip_leading_rows is 0 - Instructs autodetect that there are no headers and data
        should be read starting from the first row.
        skip_leading_rows = N > 0 - Autodetect skips N-1 rows and tries to detect headers
        in row N. If headers are not detected, row N is just skipped. Otherwise, row N is
        used to extract column names for the detected schema.
        Default value set to None so that autodetect option can detect schema fields.
    :param write_disposition: The write disposition if the table already exists.
    :param field_delimiter: The delimiter to use when loading from a CSV.
    :param max_bad_records: The maximum number of bad records that BigQuery can
        ignore when running the job.
    :param quote_character: The value that is used to quote data sections in a CSV file.
    :param ignore_unknown_values: [Optional] Indicates if BigQuery should allow
        extra values that are not represented in the table schema.
        If true, the extra values are ignored. If false, records with extra columns
        are treated as bad records, and if there are too many bad records, an
        invalid error is returned in the job result.
    :param allow_quoted_newlines: Whether to allow quoted newlines (true) or not (false).
    :param allow_jagged_rows: Accept rows that are missing trailing optional columns.
        The missing values are treated as nulls. If false, records with missing trailing
        columns are treated as bad records, and if there are too many bad records, an