Set DdlRel and Ddl objects to use Expression.Nested.Struct

[gord02]

DdlRel still uses Expression.Literal.Struct. This format is deprecated in favor of Expression.Nested.Struct and is to be replaced in the PR.

[nielspardon]

Is this something that needs to be changed in the proto definitions first? I see that DdlRel still uses Expression.Literal.Struct for table_defaults: https://github.com/substrait-io/substrait/blob/22c8d46a338e18f13badc55da95a6324cf33abf3/proto/substrait/algebra.proto#L625C1-L625C48

[nielspardon]

I see the corresponding PR for the spec was closed by you: substrait-io/substrait#877