Support Hashmaps in fields

we should have an annotation to have multivalue properties be hashmaps, that is arrays with custom keys that get preserved. there are 3 options:

• two multivalue fields with keys and values respectively. see i.e. https://github.com/symfony-cmf/symfony-cmf/pull/140 (another workaround can be to use the @PostLoad and @PreUpdate hooks to split/merge the arrays)
• serialize the array into a string field
• child nodes (performance penalty). they would need to be in a special namespace to be identifyable and have a special format. a node for key-value is a lot. if you want a document with a specific name, you should use child documents directly, not a hashmap with document entry.