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:

```rust
pub enum JobStatus {
    NotRun,
    Finished,
    Failed,
}
```

Which translates to:

```sql
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.