Proposal of EmbededDBMixins

[i8enn]

Implementation of NestedDBMixin or EmbededDBMixin for save nested pydantic models to mongo (like as EmbededDocument from MongoEngine).

• [ ] EmbededDocument as separate class
• [ ] Unpacking EmbededDocument to main model as Pydantic models
• [ ] Custom type for list of EmbeddedDocument
• [ ] Search methods for list of EmbeddedDocument
• [ ] Methods for save model from EmbeddedDocument