add support for ingesting chat contexts / chat completion messages from other libraries #255
Description
Users sometimes require interfacing with non-Mellea chat contexts / chat completion messages. We should provide parsing libraries to cast them to Mellea types so that they can be easily utilized. This functionality should at least encompass the backend / protocols that we already support.
The simplest form of this should be casting them directly into Mellea's Message type. We should investigate if it makes sense to be able to generically parse into other Mellea types.