spake2/Cargo.toml: add metadata to fix cargo-release commit messages

spake2/Cargo.toml: add metadata to fix cargo-release commit messages

This ensures that our tags and automatically-generated commits (before and
after each release, automated by 'cargo-release') refer to the correct
version strings, like "spake2-v0.1.0". The default tag would be
"spake2-0.1.0", and the commit messages wouldn't mention "spake2" at
all (which might get confusing now that we have multiple crates in a single
repo).