feat: add loader for rich text files (#3227)
### Summary
Adds a loader for rich text files. Requires `unstructured>=0.5.12`.
### Testing
The following test uses the example RTF file from the [`unstructured`
repo](https://github.com/Unstructured-IO/unstructured/tree/main/example-docs).
```python
from langchain.document_loaders import UnstructuredRTFLoader
loader = UnstructuredRTFLoader("fake-doc.rtf", mode="elements")
docs = loader.load()
docs[0].page_content
```
