Allow for user-provided schema version storage and retrivial

[Shirogaki]

Reading the documentation, the migration table is hardcoded to _sqlx_migrations with an implementation defined schema.

Since I'm already going to have a small table for unique key-value pairs, I'd like to store the version schema in that table. If possible, I think an interface like this would be ideal:

let current_schema_version: i64; // User code to fetch the current version.
let new_schema_version = migrate_from!(current_version).await?;
// User code that saves the new version somewhere.

Otherwise if reusing the Migrator struct is preferable:

let current_schema_version: i64; // User code to fetch the current version.
let new_schema_version = migrate!()
    .with_current(current_version)
    .run(&pool)
    .await?;
// User code that saves the new version somewhere.

Describe alternatives you've considered
Right now the Migrator object seem to offer a way for the user to define their own migration source with the MigrationSource trait, but it doesn't seem to offer an API to load and retrieve the current version.

[abonander]

If you want to get the current applied version, you can call Migrate::list_applied_migrations() on a connection and look at the last migration in the list.