Adding New Components, XSS treatment in the chat, other fixes (#571)

- PostgresChatMessageHistory added to allow loading memory from external
sources
- MultiQueryRetriever added which builds better queries to
Retrievers/VectorStores
- XSS protection in the chat that uses markdown
- FrontendNode refactor start
- Fix Serializable nodes that were exposing wrong base classes on
Components
- ConversationEntityMemory as the first memory with EntityStore (entity
stores coming very soon)