text: Title: [ENH] discrete signature feature transformer
  Body: It would be great to have a simple, no-bells-and-whistles signature feature transformer implemented in `sktime`.
  (Most variations like windowing or augmentations can then be obtained from this via transformation pipelines)
  
  Current implementations suffer either from overengineering (and/or unclear, unstable interfaces), or dependency on abandoned packages relying on lower python versions, see discussion in https://github.com/sktime/sktime/issues/7255
  
  However, the idea behind signatures is quite simple, they are time-ordered generalizations of moments for multivariate time series, I describe it in the following for a bivariate time series (after taking first differences).
  
  Let $(x_{1,1}, x_{2,1}) , \dots, (x_{1,T}, x_{2,T})$ be a bivariate time series.
  
  Elements of the "signature" are indexed by strings made up of the literals 1, and 2.
  I list the signature elements for the strings up to length 3, the general pattern should follow.
  
  1: the mean of the ${x_{1, i}}$, over all $i$.
  
  2: the mean of the ${x_{2, i}}$, over all $i$.
  
  11: the mean of all products ${x_{1, i}\cdot x_{1, j}}$, over all $i<j$.
  
  12: the mean of all products ${x_{1, i}\cdot x_{2, j}}$, over all $i<j$.
  
  $\ell m$ : the mean of all products ${x_{\ell, i}\cdot x_{m, j}}$, over all $i<j$.
  
  $\ell m n$: the mean of all products ${x_{\ell, i}\cdot x_{m, j}\cdot x_{n, k}}$, over all $i<j<k$.
  
  And so on. Generalisation to longer strings, or more dimensions should be clear.
  
  Due to combinatorial explosion, it typically does not make sense to compute higher than 3, or 2 for high-dimensional time series.
  
  The transformer should be parameterized in terms of an integer `degree`, that is the length of strings to include. Often, the index of the time series is included as a dimension, so for univariate time series, one gets two dimension together with the time index. This should be a boolean option, e.g., `use_index`.
  
  For column names of the output, I would simply choose the strings as above, e.g., `"1"`, `"2"`, `"12"`, and so on.
  
  The transformer is "series-to-primitives" type, the supersimple extension template should be sufficient to follow: https://github.com/sktime/sktime/blob/main/extension_templates/transformer_supersimple.py
  
  The `scitypes:transform-output` tag should change to `"Primitives".
  
  