feat(python): Add dtype argument for repeat

[stinodego]

Closes #8771

Not sure this is the right way to go, since we're now casting after the repeat call. But then again, this is also how Series.repeat was already implemented, and also similar to lit(dtype=...).

In the issue, @ritchie46 you mentioned to "cast the anyvalue before you expand", but I'm not sure how that would work.

I also made repeat(name=...) work for expressions; no reason it shouldn't.

As a side note, I don't see the point of the n argument supporting Expr input - it creates the expectation that the following will somehow work, but it doesn't:

df = pl.DataFrame({'n': [1, 2, 3]})
df.select(pl.repeat(5, n=pl.col('n')))

Results in:

shape: (1, 1)
┌─────────┐
│ literal │
│ ---     │
│ i32     │
╞═════════╡
│ 5       │
└─────────┘

Apparently only the very first value of the expression is considered? This is unexpected.

[stinodego]

I will merge this, but I'll follow up with some more related improvements/refactor.