Allow banning multiple versions of a specific crate

[leops]

This PR adds a multiple-versions boolean field to the deny list in bans, allowing the multiple-versions lint level to be set to "allow" or "warn" for all crates and overriding it to "deny" for a chosen few:

[bans]
multiple-versions = "allow"
deny = [
    { name = "naga", multiple-versions = true },
]

Settings both multiple-versions and wrappers is explicitely not supported as I don't think it would make sense, I added an error diagnostic for that specific case that unfortunately required to add a few Option and Spanned wrappers in CrateBan to display the relevant labels. The exact format of the configuration could be different anyway, I simply went with an option that seemed to require only a few changes instead of introducing a completely new deny list specifically for duplicate crates for instance

[LukeMathWalker]

We'd love to have this as well.
We are currently working around the lack of this feature by using multiple deny statements - e.g.

deny = [
    { name = "opentelemetry", version = "<0.14"},
    { name = "opentelemetry", version = ">0.14"}
]

but this requires updating the deny.toml when we update that crate.