This moves the castore.proto file into the //tvix/store/protos directory. Per-
component protos might make more sense, than a "tvix-wide" proto directory.
Change-Id: Ie728210174b041e9285a0e2ac605d715d0f0cbda
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7285
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
