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]

Is this even necessary? Will the new schema version be set to current by default? If not, is there a better way to "increment" the schema version?

[jonathanc-n]

I believe this functionality should be covered in a schema update action. It does seem nice to have an easier way to add a field. What are your thoughts? cc @liurenjie1024 @Fokko

[cmcarthur]

Thanks for taking a look @jonathanc-n. Of course, I saw your PR for schema update after I opened this 😀. I will await feedback from the others!

[ZENOTME]

I believe this functionality should be covered in a schema update action. It does seem nice to have an easier way to add a field. What are your thoughts? cc @liurenjie1024 @Fokko

+1 for this. Seems that how iceberg-java do: https://github.com/apache/iceberg/blob/97d1107d60b42d29306c663f23f869fc1d439e6b/api/src/main/java/org/apache/iceberg/UpdateSchema.java#L69