PatchTST: Add support for time features

[rshyamsundar]

Description of changes: Adding support for including time features in the PachTST model. The original PatchTST paper does not talk about time-based features and there are several ways one could include them.

Here we go with this simple approach (and maintain the simplicity of the PatchTST model):

• Similar to target time series, time series features, if provided, are converted to patches and appended to the target patches.
• Since PatchTST is encoder only model, we combine past_time_feat and future_time_feat (this dichotomy is an artefact of gluonts) and (left) shift them by prediction_length so that the features are aligned with the target input.

Added tests for the PatchTSTEstimator with features enabled.

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[kashif]

@rshyamsundar the "lag-tst" model can also be tried as it allows one to take those features naturally