update google-cloud-bigquery to use bigquery_magics package

[tswast]

The code in /~https://github.com/googleapis/python-bigquery/tree/main/google/cloud/bigquery/magics is redundant with this module. After our first release, delete that code and update google-cloud-bigquery to be backwards-compatible.

Sub-tasks:

• Update google.cloud.bigquery.load_ipython_extension to load the bigquery_magics extension instead. Raise an exception warning prompting the user to install bigquery_magics if not installed.
• [N/A] Remove /~https://github.com/googleapis/python-bigquery/tree/main/google/cloud/bigquery/magics except for google.cloud.bigquery.magics.context and google.cloud.bigquery.magics.magics.context, which will be aliases to the bigquery_magics.context object. Edit: keeping this code intact for now, but deprecating it. To be removed in a future version of google-cloud-bigquery.
• [N/A] Update unit/system tests to be minimal to verify that we're calling out to bigquery_magics now. keeping this code intact for now, but deprecating it. To be removed in a future version of google-cloud-bigquery.
• Update the magics-related "extras" in google-cloud-bigquery's setup.py to just depend on bigquery_magics.

[tswast]

googleapis/python-bigquery#1965 covers this issue. Note: it doesn't fully remove the google.cloud.bigquery.magics module, just deprecates it and updates to use this package when available.