Add snake_case rename support for Type macro

[shssoichiro]

I encountered a use case while converting a project
from Diesel to sqlx, where I had a custom Postgres enum
which included a snake case field name:

pub enum JobStatus {
    NotRun,
    Finished,
    Failed,
}

Which translates to:

CREATE TYPE job_status AS ENUM ('not_run', 'finished', 'failed');

This is likely to be a semi-common use case,
so this commit adds snake case support for enums
via the #[sqlx(rename_all = "snake_case")] attribute.

[mehcode]

Simple uncontroversial addition. 👍 And I still chuckle whenever I see the heck crate.