feat(schema): Add add_fields action to enable table schema updates

[tomighita]

Which issue does this PR close?

• Closes Add support for adding new fields to an iceberg table #2119.

What changes are included in this PR?

This PR creates an AddFieldsAction which implements the TransactionAction allowing users of the crate to add new fields to an iceberg table by updating the table schema. It also checks that the added fields are either optional or have a default value to avoid data corruption downstream.

Are these changes tested?

• Unit tests for AddFieldsAction.
• Adds one integration tests which tests that we can add data, update the table and can still read the data, ensuring backwards compatiblity.

[CTTY]

I'm not sure if this is the right direction, exposing add_fields alone in transaction layer seems very limiting. Maybe UpdateSchema can help with your use case?