Add support for setting a default migrator

[Kobzol]

This PR contains an implementation sketch for setting a default migrator for #[sqlx::test] macros, because repeating all the migrations for each such macro can produce a lot of code bloat and slow compilation times (see #4318 and https://kobzol.github.io/rust/2026/06/21/optimizing-sqlx-test-rebuild-time.html for more details).

It is configured in the config:

[migrate]
default-migrator = "crate::MIGRATOR"

When a #[sqlx::test] macro doesn't specify the migrations nor the migrator field, and a default migrator is set, it will load it the migrations from it (via a Rust path), rather than inlining the migrations in the expanded #[sqlx::test] output.

So it turns from

let mut args = ::sqlx::testing::TestArgs::new("foo::foo");
    args . migrator (& :: sqlx :: migrate :: Migrator { migrations : :: std :: borrow :: Cow :: Borrowed (const { & [:: sqlx :: migrate :: Migration { version : 20260630083325i64 , description : :: std :: borrow :: Cow :: Borrowed ("migration-1") , migration_type : :: sqlx :: migrate :: MigrationType :: Simple , sql : :: sqlx :: SqlStr :: from_static ("-- Add migration script here\nCREATE TABLE users\n(\n    id   SERIAL,\n    name TEXT\n);\n") , no_tx : false , checksum : :: std :: borrow :: Cow :: Borrowed (& [119u8 , 109u8 , 53u8 , 188u8 , 84u8 , 113u8 , 67u8 , 160u8 , 82u8 , 175u8 , 115u8 , 185u8 , 158u8 , 182u8 , 47u8 , 152u8 , 38u8 , 62u8 , 17u8 , 118u8 , 9u8 , 171u8 , 33u8 , 240u8 , 65u8 , 122u8 , 28u8 , 220u8 , 134u8 , 210u8 , 109u8 , 200u8 , 153u8 , 114u8 , 45u8 , 248u8 , 16u8 , 243u8 , 46u8 , 141u8 , 77u8 , 117u8 , 201u8 , 65u8 , 160u8 , 30u8 , 50u8 , 195u8]) , }] }) , create_schemas : :: std :: borrow :: Cow :: Borrowed (& []) , table_name : :: std :: borrow :: Cow :: Borrowed ("_sqlx_migrations") , .. :: sqlx :: migrate :: Migrator :: DEFAULT }) ; 

// potentially kilobytes of migrations embedded here

to

let mut args = ::sqlx::testing::TestArgs::new("foo::foo");
args.migrator(&crate::MIGRATOR);

I'm not sure in which config section this makes the most sense, so far I added it to the default migrate section, but maybe it should have a new dedicated test section?

I didn't write tests or document the config option yet, because I first wanted to get a vibe check whether you find the approach reasonable.

TODO:

• Tests
• Document the new config
• Allow specifying a default migrator per crate (?)

Does your PR solve an issue?

Fixes: #4318

Is this a breaking change?

No, it is a new opt-in option in the sqlx TOML config.