Enable custom trait bounds for CLI

[wfchandler]

Allow users to specify additional trait bounds on CliConfig methods.

[ahl]

I'm not a huge fan of this--the whole point of including JsonSchema as a trait bound was to satisfy this use case. If it's not useful then perhaps we remove it.

[wfchandler]

I'm not a huge fan of this--the whole point of including JsonSchema as a trait bound was to satisfy this use case. If it's not useful then perhaps we remove it.

@ahl removing the JsonSchema bound would be a breaking change for some of our projects that generate a CLI, rfd-api is a public example, but there are several internal repos as well. JsonSchema is clearly useful in some scenarios, but I think the type reflection in oxidecomputer/oxide.rs#1221 is simpler with the proc macro approach.