Implement initial-default

[Fokko]

Default values are part of the Iceberg specification V3: https://github.com/apache/iceberg/blob/main/format/spec.md#default-values

But they are handy for reading the Iceberg metadata as well. If you bump a table from V1 to V2, history is not rewritten. This is being done lazily because Iceberg tables can exceed petabytes. Therefore, if you read a V2 table, it might be V1 metadata. In this case, some fields (eg sequence numbers), need to be read with default-values:

Implementing initial-values is something that's part of V3, and also simplifies reading older Iceberg metadata as a newer version 🥳