[Enh]: Add str.pad_start, str.pad_end

[yuuuxt]

We would like to learn about your use case. For example, if this feature is needed to adopt Narwhals in an open source project, could you please enter the link to it below?

trying to replace polars with narwhals and is blocked by pad_start, pad_end

Please describe the purpose of the new feature or describe the problem to solve.

to make more use of narwhals.

Suggest a solution if possible.

https://docs.pola.rs/api/python/dev/reference/expressions/api/polars.Expr.str.pad_start.html
https://docs.pola.rs/api/python/dev/reference/expressions/api/polars.Expr.str.pad_end.html

If you have tried alternatives, please describe them below.

No response

Additional information that may help us understand your needs.

No response

[MarcoGorelli]

seems reasonable, thanks for the request

[camriddell]

@MarcoGorelli @dangotbanned can I take this one on?

[MarcoGorelli]

would love for you to do so!

[dangotbanned]

@MarcoGorelli @dangotbanned can I take this one on?

Definitely!

Be sure to check out #2598 (zfill) - since at least the pyarrow one identified what the API is called 😄

I realised that recently for #2572 and split it out already

narwhals/narwhals/_plan/arrow/functions/_strings.py

Lines 142 to 146 in 6713331

	def pad_start(
	native: ChunkedOrScalarAny, length: int, fill_char: str = " "
	) -> ChunkedOrScalarAny: # pragma: no cover
	"""Pad the start of the string until it reaches the given length."""
	return pc.utf8_lpad(native, length, fill_char)