feat: add an AddFieldsAction to transaction

[cmcarthur]

Which issue does this PR close?

• Closes Implement an AddFields operation #1174.

What changes are included in this PR?

• Adds a new AddFieldsAction which represents an addition of new fields to an existing table
• Adds a new method to Transaction add_fields which generates an AddFieldsAction
• When applied, the AddFieldsAction will register a new schema with the addition of the new fields, and set the current schema version to (schema + 1). (I am not highly confident that this is the right way to manage the schema id, though I've verified that it works in my testing. Is there a better way to mark the next schema version as the current one? Is this necessary?)

Are these changes tested?

Added tests:

Unit: in crates/iceberg/src/transaction/add_field.rs
Integration: in crates/integration_tests/tests/shared_tests/add_field_test.rs

[cmcarthur]

This was opened against a non-updated main, I will close and re-open.