fix(features): make migration support fully opt-in

[leseb]

Summary

Remove unconditional migration features from the facade and macro-support dependencies.

Migration support is now forwarded to sqlx-core and enabled database drivers only when the public migrate feature is selected. This preserves existing migration and CLI behavior while allowing no-default-features consumers to omit migration checksum dependencies.

Does your PR solve an issue?

Closes #4418

Is this a breaking change?

No intended public API break.

Users explicitly enabling migrate, as well as users relying on SQLx’s default features, retain the existing behavior. Downstream builds relying on accidental transitive activation of migration support will need to enable migrate explicitly.

Testing

Validated that:

• Runtime-only builds exclude sqlx-core/migrate, sha2, and crc.
• Macro builds with PostgreSQL, MySQL, or SQLite do not activate migration support unless requested.
• Enabling migrate activates migration support for sqlx-core and every enabled driver.
• Existing migration, macro, facade, and CLI configurations continue to compile.

Commands run include:

• cargo fmt --all -- --check
• cargo test -p sqlx-core --all-features
• cargo test -p sqlx-macros-core --all-features
• cargo test -p sqlx --lib --all-features
• cargo check -p sqlx-cli --all-features
• cargo clippy -p sqlx --no-default-features --features all-databases,macros -- -D warnings

No conventional Rust regression test was added because the regression concerns Cargo’s resolved feature and dependency graph, which cannot be inspected from a unit test after compilation. The negative and positive feature combinations were instead verified directly with cargo tree and cargo check.