feat(wfe-buildkit): add BuildKit image builder executor

Standalone crate implementing StepBody for building container images
via buildctl CLI. Supports Dockerfiles, multi-stage targets, tags,
build args, cache import/export, push to registry.

Security: TLS client certs for buildkitd connections, per-registry
authentication for push operations.

Testable without daemon via build_command() and parse_digest().
20 tests, 85%+ coverage.

wfe-buildkit/src/lib.rs

@@ -0,0 +1,5 @@
+pub mod config;
+pub mod step;
+
+pub use config::{BuildkitConfig, RegistryAuth, TlsConfig};
+pub use step::BuildkitStep;