[ENH] Add v2 interface support for DeepAR

[Soham-47]

Referencing #1992

The DeepAR model currently does not have a v2-compliant estimator/package. Unlike DLinear, SAMformer, and TFT, it does not provide a *_pkg_v2 wrapper inheriting from _BasePtForecasterV2, so it cannot be used through the v2 interface.

Should we add a v2 package for DeepAR? If so, would interfacing with the existing implementation or forking and adapting it be preferred?

CC: @fkiraly

[PranavBhatP]

Yes a v2 implementation would be welcome!