gRPC server surface for turbovec (+ a stale MSRV)

turbovec has Rust and Python surfaces. A gRPC server would let any language reach the same in-memory index. The proto's small since the payload is just vectors.

Here's my working turbovec-grpc implementation. Thumbs up and I'll open a PR for review:

Sidenote - added as second commit: gRPC needed 1.80 (tonic/axum), and checking that I noticed faer 0.20.2 already requires 1.81 while the workspace declares 1.70, so I bumped the declared MSRV to 1.81. Tested on my machine - all green.

I figured in-tree since you already have the Python bindings there, feels like a good home; hope you agree.

Thumbs up now you'll get multiple language examples and a non-slopped tutorial as a bonus. Thoughts?