We should not register a pytest plugin automaticallly, as we may
have fixtures that might override user's fixtures. Also, it might
raise import errors as in #10118.

We can still use this as a plugin ourselves by setting the following
in `conftest.py` file.
```python
pytest_plugins = ['dvc.testing.plugin']
```

* experimental: support connection strings for databases using connectorx

This adds an experimental support for connector strings using connectorx.

User has to provide config for the connection strings with:

Example:
```console
dvc config --local db_connection.pg postgres://user:pass@localhost:5432/database
```

And then, they can import using:
```console
dvc import-db --sql 'query' --conn pg
```

Check supported databases and how to configure in https://sfu-db.github.io/connector-x/databases.html.
This `db_connection` config is subject to change.

connectorx still has the same problem with dbt as before: no support for streaming,
but it'll be a bit more simpler to configure with the connection strings.

* add pandas to deps

* sqlalchemy

* dbt streaming sql

* fix test

* remove connectorx

* remove check in set

* use mode='wb' for serialization

* testing: bench: add pytest_report_header

Useful for sanity checks in logs.

* Update dvc/testing/benchmarks/plugin.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>