Add support for converting .msg files to Documents

[sjrl]

Is your feature request related to a problem? Please describe.
Recently we have had more clients want to be able to use .msg files in their RAG pipelines. The .msg format is a Microsoft email format and is not trivial to convert without the help of an external library.

Describe the solution you'd like
It would be great if we could add a MSGToDocument converter to Haystack.

Additional context
Some libraries I researched that could help with this are:

python-oxmsg (comes from the same dev we use for our PPTXToDocument converter)

• Github: https://github.com/scanny/python-oxmsg
• Docs: https://scanny.github.io/python-oxmsg/message/
• Example converter implementation using python-oxmsg by Unstrucured: https://github.com/Unstructured-IO/unstructured/blob/main/unstructured/partition/msg.py

msg-extractor (actively maintained but has a GPL-3.0 license)

• Github: https://github.com/TeamMsgExtractor/msg-extractor