feat: [py] DFtoVW -> Add weight attribute to SimpleLabel

[etiennekintzler]

Importance weight is part of the VW specification for Simple label (see https://github.com/VowpalWabbit/vowpal_wabbit/wiki/Input-format#simple) but was not implemented/forgotten in DFtoVW.SimpleLabel.

For instance, the previous SimpleLabel class could not handle this user's needs: https://stackoverflow.com/questions/65385962/how-to-convert-csv-columns-into-vowpal-wabbit-txt-input-file/65387264#65387264

I named the attribute weight but I can change it to importance if necessary.

The usage is the following:

import pandas as pd
from vowpalwabbit.DFtoVW import DFtoVW, Feature, SimpleLabel

df = pd.DataFrame({
    "y": [1, 2, -1],
    "w": [2.5, 1.2, 3.75],
    "x": ["a", "b", "c"]
})

sl = SimpleLabel(label="y", weight="w")
conv = DFtoVW(df=df, label=sl, features=Feature("x"))
conv.convert_df()

# ['1 2.5 | x=a', '2 1.2 | x=b', '-1 3.75 | x=c']

[etiennekintzler]

Not directly linked to this PR (but shorter)

[olgavrou]

lgtm :)