experimental: support connection strings for databases (#10125)

* 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

pyproject.toml

@@ -77,6 +77,7 @@ gs = ["dvc-gs==2.22.1"]
 hdfs = ["dvc-hdfs==2.19"]
 lint = [
     "mypy==1.7.0",
+    "pandas-stubs",
     "types-colorama",
     "types-psutil",
     "types-pyinstaller",
@@ -111,6 +112,8 @@ tests = [
     'pywin32>=225; sys_platform == "win32"', # optional test dependency
     'tzdata; sys_platform == "win32"',  # for testing with celery
     "dbt-sqlite", # optional dependency for testing `import-db`
+    "sqlalchemy>=1,<3", # optional dependency for `import-db`
+    "pandas>=1",
 ]
 webdav = ["dvc-webdav==2.19.1"]
 webhdfs = ["dvc-webhdfs==2.19"]