[Python] support custom target name in partition spec builder

[jun-he]

Support custom target name in partition spec builder and also fix some other issues. Additionally, add unit tests for partition spec.

[TGooch44]

Hi Jun, I think I have a very similar PR queued up on this. Let me review what you have here, and see what else needs to be added(I can rebase me PR). There is a lot of work in the partition spec that needs to be rolled out and I've been lagging on getting the PR's to the Open source

[jun-he]

@TGooch44 Thanks for the comment. Please let me know other related changes and I can add them. Also, we may ship them separately with multiple PRs.

[TGooch44]

Overall looks good. I'm not sure about the tox mypy issue for py38 when I run tox -epy38 locally on a fresh env, I don't get any errors.

[TGooch44]

Do you think it's worthwhile introducing a function that checks for redundant partitions and adds the field? Each transform method does the same block creating the partition field and then adding it. Something like this:

def check_redundant_and_add_field(self, field_id: int, name: str, transform: 'Transform') -> None:
        part_field = (PartitionField(field_id,
                                     self.__next_field_id(),
                                     name,
                                     transform))
        self.check_for_redundant_partitions(part_field)
        self.fields.append(part_field)

[jun-he]

Yep, good idea. We can actually update check_for_redundant_partitions method to include adding field.

[TGooch44]

Is this not supported?

[jun-he]

Yep, put todos for unimplemented Transforms, e.g. BucketUUID, etc. Also, need to update from_string to support unrecognized cases. In Java, it returns return new UnknownTransform<>(type, transform);
So I think it might be better to have them in a different PR.

[jun-he]

Seems the failures are from linters

  linters run-test: commands[2] | mypy --ignore-missing-imports iceberg/
  iceberg/api/transforms/transform_util.py:20: error: Library stubs not installed for "pytz" (or incompatible with Python 3.8)
  iceberg/api/expressions/literals.py:23: error: Library stubs not installed for "pytz" (or incompatible with Python 3.8)
  iceberg/api/expressions/literals.py:23: note: Hint: "python3 -m pip install types-pytz"
  iceberg/api/expressions/literals.py:23: note: (or run "mypy --install-types" to install all missing stub packages)
  iceberg/api/expressions/literals.py:23: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
  iceberg/api/expressions/literals.py:385: error: Library stubs not installed for "dateutil.parser" (or incompatible with Python 3.8)
  iceberg/api/expressions/literals.py:385: note: Hint: "python3 -m pip install types-python-dateutil"
  iceberg/api/expressions/literals.py:385: error: Library stubs not installed for "dateutil" (or incompatible with Python 3.8)

[TGooch44]

Looks good, I'll get my merge access sorted out today, and I can get this merged. We may need to do something to address the linting issue, although I'm still able to run tox on your branch and get no linting errors.