Add cross config and GitHub release workflow
Sets up cross-rs for armv6, armv7, and aarch64 Linux targets, pinned to
the 0.2.5 images for reproducibility. Adds .github/workflows/release.yml
that builds smolsonic for macOS (amd64, aarch64) and Linux (amd64,
aarch64, armhf) on tag push, packages each binary with the README,
LICENSE, and example config into a tar.gz with a sha256, and publishes
them to a GitHub release. Forces libsqlite3-sys to vendor SQLite so
cross builds don't try to link against the host libsqlite3.