Add `#[schemars(extend("key" = value))]` attribute

[GREsau]

Implements #50

• this attribute can be used to add properties (or replace existing properties) in a generated schema
• can be set on a struct, enum, or enum variant
• can have multiple key/value pairs and/or be specified multiple times - but if so, every key must be unique (otherwise, a compile error is emitted)
• key must be quoted but can be any string
• value can be any expression that produces a value implementing Serialize
• value can also be a JSON literal which can interpolate other values, e.g. #[schemars(extend("obj" = {"array": [null, ()]}))]
  • this follows the same rules as arguments to the serde_json::json!(...) macro (which this attribute uses under the hood)