Add regressor property to TSDataset

[iKintosh]

🚀 Feature Request

Add regressor property which works similar to segment property:

https://github.com/tinkoff-ai/etna-ts/blob/6c12eb6ab66c5bd1c8d5018243e13f5e32f8bce0/etna/datasets/tsdataset.py#L219-L222

Motivation

It will eliminate need for such code checks:
https://github.com/tinkoff-ai/etna-ts/blob/6c12eb6ab66c5bd1c8d5018243e13f5e32f8bce0/etna/models/prophet.py#L113

Proposal

Add regressor property to TSDataset which returns list of all regressors in dataset

Test cases

• Check that all regressors name are returned

Alternatives

No response

Additional context

No response

Checklist

• Added feature request
• Added motivation
• Added proposal