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/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "wfe-buildkit"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+homepage.workspace = true
+description = "BuildKit image builder executor for WFE"
+
+[dependencies]
+wfe-core = { workspace = true }
+tokio = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+async-trait = { workspace = true }
+tracing = { workspace = true }
+thiserror = { workspace = true }
+regex = { workspace = true }
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tokio = { workspace = true, features = ["test-util"] }