webauthn-rs-core unconditionally depends on openssl for X.509 and
signature verification. Add pkg-config and openssl to the nix build
so the package builds in the sandbox.
- Replace manual forAllSystems pattern with flake-parts.lib.mkFlake
- Use perSystem for packages and devShells definitions
- Pin rust-overlay's nixpkgs to follow main nixpkgs input
- No functional changes to package or dev shell
- Add tls-rustls feature to sqlx
- Disable default features on reqwest to avoid native-tls
- Remove openssl from flake.nix build and dev dependencies
This simplifies the build by removing the native OpenSSL dependency,
using pure-Rust TLS implementation instead.
