[plugin] Refactor lambda-build packaging into a pluggable ArchiveBackend (ZIP today, OCI-ready)

[sebsto]

Summary

Introduces a pluggable archive backend abstraction for lambda-build, mirroring
the existing build-backend design (#679). Packaging the built binaries into a
deployable artifact was hard-coded to ZIP inside Builder.swift; it is now behind
an ArchiveBackend protocol with one implementation today (ZIP, the existing code
moved verbatim) and a stubbed seam for OCI images in the future.

Internal refactor: the default behaviour is unchanged (ZIP), no public Swift API
changes, and the output ZIP layout / path is identical.

What changed

• ArchiveBackend protocol (ArchiveBackends/ArchiveBackend.swift) — owns what
  kind of artifact is produced from the built binaries. It's the packaging
  counterpart to BuildBackend (which owns how the binaries are built); Builder
  selects and sequences the two.
• ZipArchiveBackend (ArchiveBackends/ZipArchiveBackend.swift) — the existing
  Builder.package(...) body, moved unchanged, with zipToolPath injected via init.
• ArchiveFormat enum (ArchiveFormat.swift) — the parsed --archive-format
  value (zip | oci), mirroring CrossCompileMethod: parse(...) (defaults to
  zip, case-insensitive), isSupported (oci → false), and a new
  BuilderErrors.unsupportedArchiveFormat case.
• --archive-format zip|oci flag — parsed into BuilderConfiguration, with a
  makeArchiveBackend() factory (following the same on-configuration factory
  convention as makeCrossCompileBackend()). oci is recognised but throws a
  "not yet supported" error. Help text and the verbose config dump updated.
• Builder.build(...) now ends with configuration.makeArchiveBackend().archive(...)
  instead of the inline package(...) method (which is removed).
• Directory rename: Backends/ → BuildBackends/, so the two families read as
  parallel siblings: BuildBackends/ and ArchiveBackends/.

Adding OCI later

Add OCIArchiveBackend: ArchiveBackend under ArchiveBackends/, then flip the oci
case in ArchiveFormat.isSupported and makeArchiveBackend(). No other changes.

Testing

• New ArchiveBackendTests.swift: ArchiveFormat.parse (default/case-insensitive/
  oci-unsupported/unknown), makeArchiveBackend() selection (zip → ZipArchiveBackend),
  and ZipArchiveBackend.archive run against a temp build dir asserting the .zip and
  relocated bootstrap are produced.
• All AWSLambdaPluginHelperTests pass (90 tests); swift build and
  swift format lint --strict clean.
• End-to-end: --archive-format oci fails fast with the stub error; zip/default
  produce a byte-for-byte identical artifact to before.