Split `iceberg/src/spec/values.rs` into multiple files

[manuzhang]

iceberg/src/spec/values.rs is already around 4k lines long, making it hard to read. Given there are more values to be added, we'd better split it into multiple files. cc @mbutrovich

[mbutrovich]

Thank you! I'll take a look at this one. A committer can feel free to assign to me.