New Phase 1 modules should follow the existing pattern of referencing
ADR numbers in module-level doc comments for discoverability, matching
the style in transport/mod.rs.
ForwardingAction, TargetPattern, ForwardingRule, OperationType,
InterfaceConfig, InterfaceKind, DynamicConfig, and CallError are all
likely to gain variants/fields in future phases. Marking them
#[non_exhaustive] now prevents downstream breakage when new
variants/fields are added. Added constructor methods for types that
are constructed from other crates.