Add Postgres-only SQLx bindings crate

[caniko]

Summary

This adds a small Postgres-only SQLx binding crate for SeaQuery.

The existing sea-query-sqlx crate is intentionally database-agnostic, but that also means its manifest has to mention every SQLx backend it can support. Cargo records optional dependencies from package manifests in downstream lockfiles, so a project that only enables Postgres can still end up with unrelated SQLx packages such as sqlx-mysql in Cargo.lock.

That is awkward for projects that keep a tight dependency/audit surface. In our case, the MySQL edge pulls in rsa even though no MySQL code is compiled or used.

What changed

• Added sea-query-sqlx-postgres, a Postgres-only companion crate.
• Kept the familiar SqlxBinder and SqlxValues shape so callers can continue to build a query and pass the values into SQLx.
• Wired the crate directly to sqlx-core and sqlx-postgres, without depending on the top-level sqlx facade.
• Preserved the common SeaQuery value feature flags for JSON, UUID, chrono, decimals, time, IP networks, MAC addresses, and Postgres arrays.

One limitation is called out in the implementation: pgvector's SQLx integration currently depends on the sqlx facade, so this crate does not wire that integration yet. I kept that separate from the initial change so this PR can stay focused on the dependency shape.

Why this shape

I first tried changing sea-query-sqlx itself to use direct SQLx driver crates, but that does not solve the lockfile issue. Cargo still records optional dependencies listed by the package, even when those features are not enabled. A separate per-driver package avoids that entirely because a Postgres-only consumer never sees the MySQL dependency in the first place.

Checks

• cargo check --manifest-path sea-query-sqlx-postgres/Cargo.toml --no-default-features --features with-json,with-uuid,with-chrono,with-rust_decimal,with-bigdecimal,postgres-array,runtime-tokio-rustls
• cargo check --manifest-path sea-query-sqlx-postgres/Cargo.toml --all-features
• cargo fmt --manifest-path sea-query-sqlx-postgres/Cargo.toml -- --check
• Verified in a temporary downstream crate that enabling the new Postgres crate does not add sqlx, sqlx-mysql, sqlx-macros, or rsa to Cargo.lock.

[caniko]

Marking as ready for review. @tyt2y3 @Huliiiiii @Expurple — would appreciate any thoughts on the per-driver crate shape vs. trying to slim sea-query-sqlx itself. Happy to iterate on naming or structure.