Add resolver version to cargo metadata

[sunshowers]

Describe the problem you are trying to solve
At the moment it appears that the resolver version isn't exposed in cargo metadata. It would be great to have that information so that tools know which version of the resolver is in use.

Tested by adding

cargo-features = ["resolver"]

[workspace]
resolver = "2"

to a workspace's Cargo.toml and then running cargo +nightly metadata | less, then searching for the string resolver in it.

Describe the solution you'd like
A top-level field in the cargo metadata JSON that has the resolver version in it.

Notes
Tested with cargo 1.51.0-nightly (c3abcfe8a 2021-01-25), which appears to be the current nightly (I might be wrong though!)