Remove `initial_change` when dealing with table updates

[kevinjqliu]

Closes #864

Identified in #864, TableMetadata is initialized with the default Pydantic object for schema, partition_spec, and sort_order, which does not play well with table updates. Specifically, the initial_change field is an implementation detail of pyiceberg and does not play well when interacting with the REST API. Table update objects from the REST API does not understand this field. We can safely remove initial_change by modifying the logic for dealing with table updates.

[kevinjqliu]

@HonahX do you mind taking a look at this when you get a chance?

[HonahX]

Hi @kevinjqliu I ran a few experiments and found that removing initial_change would be challenging unless we can temporarily disable Pydantic’s validators during update_table_metadata. Fortunately, I found that Pydantic’s model_construct can help bypass the validators in this context.

I’ve implemented this approach and created a draft PR: https://github.com/apache/iceberg-python/pull/1219. I’d love to hear your thoughts on it!

[kevinjqliu]

Closing this in favor of #1219