Frozen dataclass `StatementProperties`

[jlrobins]

Make a frozen dataclass for statement properties which holds typesafe fields, possibly type-converted at construction time from convienent types (say, for timedelta -> string representation).

Something like:

@dataclass(frozen=True, kw_only=True)
class StatementProperties:
    """Typed, validated view over a curated subset of Flink SET options.

    Unset fields (None) are omitted entirely; only explicitly set options are emitted, so this
    never fights the driver-owned overlay (catalog/database/snapshot.mode) or pins a server default.
    An `extra` dict is the escape hatch for options this class does not yet model.
    """
    snapshot_write_mode: SnapshotWriteMode | None = None
    state_ttl: timedelta | None = None
    scan_startup_mode: ScanStartupMode | None = None
    local_time_zone: str | None = None
    extra: PropertiesDict = field(default_factory=dict)

    def to_properties_dict(self) -> PropertiesDict:
        props: PropertiesDict = dict(self.extra)
        if self.snapshot_write_mode is not None:
            props[SNAPSHOT_WRITE_MODE] = self.snapshot_write_mode.value
        if self.state_ttl is not None:
            props[STATE_TTL] = _to_flink_duration(self.state_ttl)  # timedelta -> "3600 s"
        # ...
        return props

_resolve_properties() would notice if it was passed one of these, and if so, then would downgrade to PropertiesDict via .to_properties_dict() then continue to validate as currently done.