feat: generate openrpc definitions

[Frando]

• basics working
• switch to turn it on
• make use of schema-objects in https://spec.open-rpc.org/#components-object instead of duplicating types in responses, without them generation of types would be much much harder.
• cyclic references are broken (generate an invalid reference): example: struct Node {children: Vec<Node>} (seems to be a related issue: Self-referencing a type ends up creating a new definition GREsau/schemars#175)

[Frando]

I think this is basically good to go, but I want to add a feature flag for it still to not force the schemars dependency if not needed

[link2xt]

I have created a follow-up PR to turn schemas into references: #51.

There I modified examples/axum/openrpc.json with an example of how to reference schema objects. You can also look at the default example at https://playground.open-rpc.org/ for an example of using references internal to OpenRPC document.

[link2xt]

Now references are used in this PR, definitions are generated into #/components/schemas/....

[link2xt]

I think this is basically good to go, but I want to add a feature flag for it still to not force the schemars dependency if not needed

I added openrpc feature.