GRAMEX-186 ⁃ SARIMAX: `start` and `end` forecast parameters

[jaidevd]

MLHandler + SARIMAX needlessly expects start and end of the forecast window as query parameters. This is especially unnecessary when there is exogenous variable.

Since the exogenous data is a timeseries in itself, the forecast window is simply the span of this timeseries. No need to expect query parameters.

┆Issue is synchronized with this Jira Bug