Compile-time query support

[bobozaur]

Right now the driver does not support the sqlx-cli or the compile-time query macros due to the sqlx incorporated drivers being hardcoded in the API that decides what driver to use.
This is understandable, given that a proper plugin system would be required to allow 3rd party drivers to be used in the same manner.

There are the inventory and linkme crates but each has its caveats.

• inventory "abuses" life before main which isn't really well defined/supported in Rust and seems frail.
• linkme seems like a better solution but the platform support seems to be more limited

Nevertheless, some change must first happen in sqlx before any 3rd party driver is able to, firstly, implement the QueryDriver API and, secondly, be seamlessly plugged in so that the compile-time query checking can actually use the driver.