make `version=` in `plugins {}` allow semver contraints

[j-lanson]

Currently, we have to specify a version for a plugin in the plugins {} which doesn't work well for our config_to_policy conversion, since we don't want to have to update that code every time any of our plugins gets a new version.

Furthermore, it would be generally useful to offer users a way to just get the newest version of a plugin. We should make version= optional in the policy file and grab the newest version when selecting from the plugin's download manifest.

[alilleybrinker]

It's probably better to permit version constraints like the ones offered in Cargo or NPM. Most software ecosystems which permit version constraints have learned over time that wildcard constraints ("just get latest") cause more problems than they solve, because they don't offer any ability for the user to manage breakage.

Cargo and NPM have slightly different operators:

• Cargo: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html
• NPM: https://github.com/npm/node-semver#advanced-range-syntax

[j-lanson]

Use semver crate to achieve this