chore(deps): bump prost to 0.12

[waynexia]

closes https://github.com/tikv/grpc-rs/issues/635

[waynexia]

I've checked this with cargo check --workspace -F prost-codec in my env. PTAL @BusyJay

[BusyJay]

You need to regenerate all sources after upgrading prost-build.

[waynexia]

Thanks for your guide. I've run cargo xtask codegen and pushed the changes 👍

[waynexia]

Have the following commands passed:

cargo clippy --all -- -D clippy::all && cargo clippy --all --no-default-features --features prost-codec -- -D clippy::all
cd proto && cargo build --no-default-features --features prost-codec

PLTA again 🥲

[BusyJay]

Should be covered by #648. Thanks for your contribution!