Add #[elastic(ignore)] attribute to derive DocumentType

[KodrAus]

When deriving a document type, it should be possible to exclude fields from the PropertiesMapping implementation without ignoring them in serialisation:

#[derive(ElasticType)]
struct MyType {
    id: i32,
    title: String,
    #[elastic(ignore)]
    tags: BTreeMap<String, Value>,
}

// Manual serialisation where `tags` are flattened onto the parent object, instead of being nested

One usecase for this would be having fields that are serialised differently by some manual serde implementation, and shouldn't be included in the mapping.