chore: Fix `IntoSchema` typing, allow to pass `Sequence[tuple[str, IntoDType]] | SupportsKeysAndGetItem[str, IntoDType]`#3266

FBruzzesi wants to merge 12 commits intomainnarwhals-dev/narwhals:mainfrom

chore/into-schema-overhaul-redonarwhals-dev/narwhals:chore/into-schema-overhaul-redoCopy head branch name to clipboard