[ENH] add BaseSeriesTransformer - Moving Average Transformer

[Datadote]

Reference Issues/PRs

• Still in progress
• Part of #1920

What does this implement/fix? Explain your changes.

• Add smoothing filter MovingAverageTransformer
• Add tests - TODO

Does your contribution introduce a new dependency? If yes, which one?

• TODO

Any other comments?

• TODO