**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).


from typing import TYPE_CHECKING, Any, Sequence
from google.api_core.exceptions import BadRequest, Conflict
from google.api_core.retry import Retry
from google.cloud.bigquery import (
    DEFAULT_RETRY,
    CopyJob,
    ExternalConfig,
    ExtractJob,
    LoadJob,
    QueryJob,
    SchemaField,
)
from google.cloud.bigquery.table import EncryptionConfiguration, Table, TableReference
from airflow import AirflowException
from airflow.models import BaseOperator
from airflow.providers.google.cloud.hooks.bigquery import BigQueryHook, BigQueryJob
from airflow.providers.google.cloud.hooks.gcs import GCSHook
from airflow.providers.google.cloud.links.bigquery import BigQueryTableLink
from airflow.providers.google.cloud.triggers.bigquery import BigQueryInsertJobTrigger
if TYPE_CHECKING:
    from airflow.utils.context import Context
ALLOWED_FORMATS = [
    "CSV",
    "NEWLINE_DELIMITED_JSON",
    "AVRO",
    "GOOGLE_SHEETS",
    "DATASTORE_BACKUP",
    "PARQUET",
]
class GCSToBigQueryOperator(BaseOperator):