(BLOCKED: merge after 0.9.0) Add example for `jiff-sqlx` using `sqlx.toml`

[abonander]

Created this example to add to #3383, but it won't compile until a new version of jiff-sqlx is released targeting 0.9.0 because Cargo won't unify the crates.io version of sqlx, that jiff-sqlx depends on, with the path dependency.

cc @burntsushi context for https://github.com/BurntSushi/jiff/pull/240#discussion_r2040401240

The diff is a mess since this has all the commits from #3383 that have yet to be merged to main. Only the last commit is relevant to this PR.