support extension attribute

[ahl]

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It would be useful to be able to annotate a container with something like #[schemars(extension = "x-foo-bar")] and have those values appear in SchemaObject::extensions. I'd be happy to submit a PR if you'd be interested in it.

[GREsau]

This makes sense to me, except that extension values would need both a name and a value, so I imagine it would be more like #[schemars(extension("x-foo-bar" = "baz"))].

I'd just need to decide two main things:

1. Whether the keys should be quoted, i.e. "x-foo-bar" = "baz" vs x-foo-bar = "baz". I think they would need to be quoted, mainly because parsing something like x-foo in syn would be quite difficult as it's not a valid rust identifier While x- prefixes aren't required in JSON schema extension attributes, they do seem to be fairly common in practice, so they'll need to be supported.
2. What would a valid value be - a string literal, or any literal, or maybe any expression? Allowing an expression would be nice, so you could pass it a global variable or a function that returns anything implementing serde::Serialize - this would look like: #[schemars(extension("x-foo-bar" = something()))].

[ahl]

1. Agreed on quotes
2. I like the idea of specifying an expression

I'd be happy to submit a PR if you'd be interested