Confine all objects to the apalis schema (generate_ulid, _sqlx_migrations, pgcrypto currently leak into public)

[tysen]

Hi - I really like apalis so thanks for making it! A couple issues I think would be nice to get in before your 1.0 release:

Every object apalis creates is apalis.-qualified (apalis.jobs, apalis.workers, apalis.get_job, …) except:

• CREATE OR REPLACE FUNCTION generate_ulid()
• CREATE EXTENSION IF NOT EXISTS pgcrypto

There's one more, which is _sqlx_migrations from sqlx. This is unfixable in sqlx 0.8, but the recently released 0.9 allows for an sqlx.toml file containing:

create-schemas = ["apalis"] — creates the schema before the migrations table
table-name = "apalis._sqlx_migrations" — schema-qualified migrations table

The sqlx.toml file would need to be in this crate - downstream users, like me, couldn't do this. This of course requires upgrading to sqlx 0.9 but that also seems like a good thing to do before apalis 1.0.