public Optional<String> getIdKeyword() {
        return Optional.ofNullable(idKeyword);
    }
Gets the "$id" keyword of the document.

@return Returns the optionally defined $id.
@see <a href="https://json-schema.org/latest/json-schema-core.html#rfc.section.8.2">$id</a>
smithy-jsonschema/src/main/java/software/amazon/smithy/jsonschema/SchemaDocument.java