Annotation for example values

[ralpha]

Adding support for examples.

Currently strings will have an example of:

[
  {
    "list_of_int": [
      0
    ],
    "id": 0,
    "name": "string",
    "full_name": "string"
  }
],

Where strings are always "string" and int is always 0
If we add examples we can give some more info.

Maybe with a syntax like:

#[schemars(example = "This is the example string")]

Used like this:

#[derive(Serialize, Deserialize, JsonSchema)]
pub struct Version {
    #[schemars(example = "1.0.0")]
    pub code: String,
    #[schemars(example = "First release of our crate/application.")]
    pub description: String,
}

This will result in something like:

{
  "code": "1.0.0",
  "description": "First release of our crate/application."
}

I don't know how this can also be done for int, but I think that is less of an issue.
For enums the first value is converted to string and that is used. So that does not have to change I think.

This issue is a repost of: GREsau/okapi#11

[GREsau]

Duplicate of #23 which was implemented in schemars v0.7.6