Extra method-level arguments for Metadata/set
(draft-ietf-jmap-metadata-01 §3.1).
Draft-01 defines no method-specific args on Metadata/set beyond the
RFC 8620 §5.3 standard set; this struct carries only the vendor /
site / private-extension extra flatten field. The if_in_state
argument is passed as a positional parameter to metadata_set,
mirroring the canonical email_set shape.
Catch-all for vendor / site / private extension fields. Preserves
unknown fields across deserialize/serialize round-trip per workspace
extras-preservation policy (see workspace AGENTS.md).
Keys MUST NOT collide with the standard RFC 8620 §5.3 arg names
(accountId, ifInState, create, update, destroy);
metadata_set will silently retain the typed-field value if an
extras key collides.