Allow migrator to have multiple sources for migrations

[kirillsalykin]

Is your feature request related to a problem? Please describe.
I've tried to use underway crate (https://github.com/maxcountryman/underway) which have its sql migrations and runs them using sqlx.
I do have my own migrations which I also run via sqlx.
Both migrators fail because it detects missing migrations - my misses underway migrations and underway migrator misses mine.

    underway::MIGRATOR.run(&pool).await.unwrap();
    sqlx::migrate!().run(&pool).await.unwrap();

Describe the solution you'd like
I'd like be able to provide multiple sources to migrator so it can run migrations from the crates w/o issues and there is no need to have ignore-missing setting.

sqlx/sqlx-core/src/migrate/migrator.rs

Line 74 in 028084b

pub async fn new<'s, S>(source: S) -> Result<Self, MigrateError>

Would be nice if a crate can expose its migrations as additional source.

Describe alternatives you've considered
Crate copies migrations into my project migrations folder.

Additional context
maxcountryman/underway#21

[abonander]

I think multiple sources in one migrator is a huge footgun and we will never support it. This has come up before.

However, an explicit goal of #3383 is to allow relocating the migrations table and supporting multiple migrators, with the idea that each application using a database could own its own schema.

[kirillsalykin]

clear, thanks for clarification.

[cboecking]

Will you kindly tag me when you need testers? Specifically for #3383

I maintain an ERP (where many 3rd parties contribute in different ways). I need each party to be able to contribute independently. I would love to apply my scenarios to this feature.

Chuck B