feat(wfe-buildkit): rewrite to use own generated protos (tonic 0.14)

Replaced third-party buildkit-client git dependency with
wfe-buildkit-protos generated from official moby/buildkit protos.

Direct ControlClient gRPC calls: SolveRequest with frontend attrs,
exporters, cache options. Daemon-local context paths for builds
(session protocol for remote transfer is TODO).

Both proto crates now use tonic 0.14 / prost 0.14 — no transitive
dependency conflicts. 95 combined tests, 85.6% region coverage.

wfe-buildkit-protos/build.rs

@@ -1,8 +1,6 @@
 use std::path::PathBuf;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let buildkit_root = PathBuf::from("vendor/buildkit");
-
     // Use Go-style import paths so protoc sees each file only once
     let proto_dir = PathBuf::from("proto");
     let go_prefix = "github.com/moby/buildkit";