feat: improve dataSource.execute to support more flavors

[bajtos]

Add support for the following variants:

• execute(collection, command, ...params, options) (MongoDB)
• execute(...params) (forward-compatibility & other databases)

This is a follow-up for #1855, which added type descriptions for execute variants that turned out to be rejected by the actual (runtime) implementation. See also loopbackio/loopback-next#3342

The changes is pushing the responsibility for processing optional arguments ((cmd), (cmd, args), (cmd, args, options)) to the connector. This should be fine because the base SqlConnector already handles optional arguments (see lib/sql.js#L551-L563), and we weren't really supporting non-SQL execute before.

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• Commit messages are following our guidelines

[bajtos]

coverage/coveralls — Coverage decreased (-0.0005%) to 84.722%

I believe this is caused by reducing the number of lines in lib/datasource.js - i.e. the number of lines executed by our tests is staying the same, but the total number of lines is lower now, therefore the coverage ratio went slightly down. Nothing to worry about.

[jannyHou]

👍