feat(wfe-buildkit, wfe-containerd): add container executor crates

Standalone workspace crates for BuildKit image building and containerd
container execution. Config types, YAML schema integration, compiler
dispatch, validation rules, and mock-based unit tests.

Current implementation shells out to buildctl/nerdctl — will be
replaced with proper gRPC clients (buildkit-client, containerd protos)
in a follow-up. Config types, YAML integration, and test infrastructure
are stable and reusable.

wfe-buildkit: 60 tests, 97.9% library coverage
wfe-containerd: 61 tests, 97.8% library coverage
447 total workspace tests.

wfe-buildkit/Cargo.toml

@@ -19,4 +19,6 @@ regex = { workspace = true }
 
 [dev-dependencies]
 pretty_assertions = { workspace = true }
+tempfile = { workspace = true }
 tokio = { workspace = true, features = ["test-util"] }
+tokio-util = "0.7"