Experiment with manual FFI and protobuf

[GregoryConrad]

Two main reasons why:

• currently we are passing Vec<String> over ffi for document lists, where these strings are actually just JSON-encoded Map<String, String>. This clearly is suboptimal and protobuff and rinf would likely help here.
• FRB requires version pinning for libraries. This means I need to make a new release of Mimir for every FRB release and user may not always have the latest dependencies. Rinf may not require version pinning which would be great for users to always have up to date dependencies.

[GregoryConrad]

I am not keen to use rinf after all the recent controversy surrounding it, so I may instead just take a look into how hard it'd be to implement everything manually with FFI when native assets role around.

[GregoryConrad]

Might as well just stick with FRB.