Adds schemars support and forwards dependency features conditionally

[StefanTerdell]

This PR adds optional support for schemars through a feature flag. It also makes use of the optional dependency feature pattern to forward activation of the serde feature to - and schemars-related features for - either and indexmap when those are active as well.

Hope you like it :)

[StefanTerdell]

@fosskers Sorry about tagging, but not sure how notifications work on GH myself TBH :) Any thoughts on this?

[fosskers]

3 weeks of silence from me definitely warrants a ping, yes.

Since this is feature-gated it's basically fine, but my main concern is whether schemars is flash-in-the-pan and not something as permanent as say serde is. I wouldn't to add this just to have to rip it out again in a year when the new cool thing appears. Happy to be convinced otherwise.

[StefanTerdell]

Well, it's been around for about a decade and is the basis of most Open Api generator libs I've used so far. It's also getting 15m monthly downloads, if that means anything :)

[StefanTerdell]

I made a similar pull request for IpNet that was just merged and released: krisprice/ipnet#67.

IpNet is used by sqlx among others, so I hope this helps make the case for the wide(ning) adoption of schemars throughout stable crates in the ecosystem :)

I am using my fork as a git dependency for now so there's really no rush on my end, just mentioning it to provide some more context.

[MOZGIII]

schemars is pretty widely-adopted by now; this is a to-go JSON schema tooling in rust.