`tower` builds are now failing on CI because Tokio v1.17.0 bumped MSRV
to 1.49.0. This branch updates `tower`'s MSRV to 1.49.0 to track Tokio's
MSRV. I also added nicer documentation of the MSRV based on Tokio's, and
added the `rust-version` Cargo metadata to the `tower` crate's
`Cargo.toml`.

Note that `tower-service` and `tower-layer` can technically continue to
support much earlier Rust versions than `tower` can, since they don't
depend on external crates and are very small. We could consider testing
separate, older MSRVs on CI for those crates individually. I didn't do
that in this PR, though, because I wasn't sure if this was worth the
effort and I just wanted to get CI passing again.