Add QueryExecutor trait

[CodesInChaos]

The trait enables writing functions that don't care if they're executed in a transaction or directly on a client.

Handling of Send needs to be discussed. Currently I'm not constraining anything to Send, but perhaps the returned futures need to be, but that might require adding constraints to some of the generic arguments (e.g. QueryResult) as well.

Closes #339

[edgedb-cla[bot]]

All commit authors signed the Contributor License Agreement.

[aljazerzen]

This looks good.

For reference, postgres crate has similar trait, named GenericClient. I think that QueryExecutor is a better name. Other db clients that I've checked (mysql, tiberious, sqlite) don't have such traits.

[aljazerzen]

I think we'd need to bump MSRV from 1.72 to 1.75 for "async fn returning impl Trait".

This is a reasonable, since 1.75 is 8 versions before current stable (1.80.1).