feat(compose): parse credential_spec, isolation, provider, use_api_socket, models

[Jaro-c]

Standard compose keys currently land in unknown/extensions with a noisy "unknown key" diagnostic, so users cannot tell a no-op from a typo. Add parsing (silently accepted, or warned where podman has no equivalent):

• credential_spec (service-level, GMSA) — absent from types/service.rs.
• isolation (service-level, distinct from build.isolation) — absent.
• provider (v2.36, external OCI runtime) — absent.
• use_api_socket (v2.37.1) — absent.
• models (v2.38 top-level element) — absent from ComposeFile (types/mod.rs).
  Additive only; no behaviour change for existing files.

[Jaro-c]

Fixed in #497 (squashed to develop).