Adds the extra_components metadata, git2 as an optional dep behind the
`git` feature (default-features off -> no openssl-sys/libssh2-sys), and a
git_smoke bin gated on it. libgit2-sys/libz-sys run in system mode against
fake pkg-config files (empty Libs) so they emit no link flags -- symbols
come from the esp-idf component. Proven on device: git2 version + a blob
SHA1 through the mbedTLS backend.
