feat: implement FromRef for State into sqlx pool

[arcstur]

this allows accessing the pool with axum's state extractor in handlers

solves #41

[connec]

Looks great, and thank you so much for the PR!

I'll try to make a release soon, but I also want to remove all the spurious cargo features from the crate so it might take a few days. You should be able to use it from git 'til then at least :)