Introduce a SQLConnectionDataSet

[BenjaminLevyQB]

Description

Split from #879

Creates a new SQLConnectionDataSet class that allows users to loads a sqlalchemy Connection object using credentials, which they can then manipulate within a node. This can allow users to do significant amounts of data manipulation in SQL (e.g. on a remote server) without having to load the result into memory via Pandas.

Development notes

• Created new class that only implements the _load method
• Added tests in new tests/sql directory

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change and added my name to the list of supporting contributions in the RELEASE.md file
• Added tests to cover my changes

Notice

• I acknowledge and agree that, by checking this box and clicking "Submit Pull Request":

• I submit this contribution under the Apache 2.0 license and represent that I am entitled to do so on behalf of myself, my employer, or relevant third parties, as applicable.

• I certify that (a) this contribution is my original creation and / or (b) to the extent it is not my original creation, I am authorised to submit this contribution on behalf of the original creator(s) or their licensees.

• I certify that the use of this contribution as authorised by the Apache 2.0 license does not violate the intellectual property rights of anyone else.

[datajoely]

Unfortunately I think need to close this for reasons outlined here.

[BenjaminLevyQB]

Closed due to above discussion