allow string transforms in add_field()

[Federico-Vichachi]

Closes #1882

Rationale for this change

This change allows the add_field method to accept str values as transform inputs, in addition to Transform instances. This improves developer experience by supporting more flexible input formats when defining transformations, and is consistent with similar string-based input handling elsewhere in the codebase.

Are there any user-facing changes?

No user-facing changes. This is an internal API enhancement that improves developer flexibility when working with partition transforms.

[Fokko]

Thanks for picking this up @Federico-Vichachi I left some small comments, and it would be nice to add a test.

It would be good to check locally if the tests pass using make lint and make test.

[Fokko]

😉

Suggested change

	transform: Union[str, Transform[Any, Any]], # Aceptamos strings o Transform
	transform: Union[str, Transform[Any, Any]], # We accept strings or Transform

[Fokko]

I think we need to revert this to make the linter happy:

Suggested change

	def add_identity(self, source_column_name: str) -> UpdateSpec:
	def add_identity(self, source_column_name: str) -> UpdateSpec:

[Federico-Vichachi]

Hi @Fokko, I appreciate your corrections and will include them. Regarding the test, I was trying to run the following one but had problems installing the project's dependencies with poetry install. This is the test I want to run in test_partition_evolution.py.
update_spec = table.update_spec()
update_spec.add_field("example_col", "identity")
update_spec.commit()
assert "example_col" in table.schema().fields`