Add example for total manual implemenation of `JsonSchema`

[ralpha]

trafficstars

I was looking for an example of a totally manual implementation of JsonSchema but could not find this easily so created one by combining a few different things. Note that there does exists an example that might fit for easier cases: https://github.com/GREsau/schemars/blob/master/schemars/examples/custom_serialization.rs This did not cover in my case, so needed to implement the trait manually.

Here is my code (don't have time to create PR right now):

#[derive(Clone, Copy, Debug)]
pub struct Permission {
    pub read: bool,
    pub write: bool,
    pub delete: bool,
}
/// Manual implementation of JsonSchema because it need to be similar to `String`.
impl JsonSchema for Permission {
    fn schema_name() -> String {
        "Permission".to_owned()
    }

    fn json_schema(_: &mut SchemaGenerator) -> Schema {
        SchemaObject {
            instance_type: Some(InstanceType::String.into()),
            format: None,
            string: Some(Box::new(StringValidation {
                max_length: Some(3),
                min_length: None,
                // Regex for permissions
                pattern: Some("^r?w?d?$".to_owned()),
            })),
            metadata: Some(Box::new(Metadata {
                description: Some(
                    "Permissions given to a specific object or user.<br/>
                    This string can contain the following permissions:<br/>
                    - Read (`r`): Entity has read permissions,<br/>
                    - Write (`w`): Entity has change/upload/add permissions,<br/>
                    - Delete (`d`): Entity has permission to delete items,<br/>
                    The letters that are present in the string are the given permissions.<br/>
                    <br/>
                    Example: \"rw\" is equal to read and write permissions.<br/>
                    But delete permissions are not granted."
                        .to_owned(),
                ),
                examples: vec![json!("rw"), json!("rwd"), json!("r"), json!("")],
                ..Default::default()
            })),
            ..Default::default()
        }
        .into()
    }

    fn is_referenceable() -> bool {
        false
    }
}

impl Serialize for Permission {
    // ...
}

impl<'de> Deserialize<'de> for Permission {
    // ...
}

Hope this helps some people out. Might be a good idea to add this example to this page: https://docs.rs/schemars/0.8.7/schemars/trait.JsonSchema.html Or a link to it, also the other example would be nice.