Add threadId and config getters to PromiseConnection

[trevorr]

These properties are documented in mysqljs/mysql [config] [threadId] and typed in types/mysql2 but not directly exposed in PromiseConnection.

[sidorares]

Thanks @trevorr ! can you add simple unit test for this?

[trevorr]

I feared you might ask that, as I'd have to downgrade from MySQL 8 to run the tests. 😉 However, your request is fair, so here you go.

[sidorares]

I have previous previous experience of accidentally removing / breaking the code that is not used often and not realising that for many month :)

[sidorares]

I'd have to downgrade from MySQL 8 to run the tests.

I use mysql in local docker to test different versions