Support user_agent via connect_args or url query parameters

**Is your feature request related to a problem? Please describe.**

It's frustrating to manually create client objects with `user_supplied_client` just to be able to customize the `user_agent`.

**Describe the solution you'd like**

I'd like `user_agent` to be configurable via `connect_args` when invoking `sa.engine.create_engine`. Since the`Client` is automatically created as part of `create_connect_args`, any user-provided `connect_args` can't currently be incorporated.

One solution would be to defer the creation of the `Client` object until `BigQueryDialect.connect` so that all `connect_args` (both from `create_connect_args` and the user) can be incorporated.

However, as a smaller iteration to allow control over this, it would be helpful to be able to specify the `user_agent` string as part of the URL so that `parse_url` (in the context of `create_connect_args`) can extract it as a `ClientInfo` object and pass it to `_helpers.create_bigquery_client` for automatic client creation.

https://github.com/googleapis/python-bigquery-sqlalchemy/blob/f363b87f7587c21ad32501e17cb4d9987fdd5d5d/sqlalchemy_bigquery/base.py#L1170-L1177

**Describe alternatives you've considered**

Today, our only option seems to be `user_supplied_client` and creating our own `client` – and `bq_storage_client` – with appropriate `ClientInfo` populated.

**Additional context**
Add any other context or screenshots about the feature request here.