support saving and loading forecast objects

[jmoralez]

Description

Adds the following methods:

• TimeSeries.save
• TimeSeries.load
• MLForecast.save
• MLForecast.load
• DistributedMLForecast.save
• DistributedMLForecast.load

Along with the following extras:

• aws: save to S3
• gcp: save to Google Cloud Storage
• azure: save to Azure Data Lake

Fixes #288

Checklist:

• [x] This PR has a meaningful title and a clear description.
• [x] The tests pass.
• [x] All linting tasks pass.
• [x] The notebooks are clean.

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