longest_streak_above/below_mean uses deprecated struct fields

[david-waterworth]

longest_streak_above_mean and longest_streak_below_mean use the polars rle function that returns

Expression of data type Struct with fields `len` of data type UInt32 and `value` of the original data type

functime expects length rather than len and value rather than values so fails on latest polars release

functime/functime/feature_extractors.py

Line 998 in ef23263

result = y.filter(y.struct.field("values")).struct.field("lengths").max()

functime/functime/feature_extractors.py

Line 1027 in ef23263

result = y.filter(y.struct.field("values")).struct.field("lengths").max()

I'm happy to create a pull request once #245 gets merged