Make inputs directly from a dict

[Deseaus]

Currently, the only way to make inputs with constraints is to serialize the information into a JSON string and call make_input_from_json. This PR adds an option to make these inputs directly with a dict object, making it easier to call from Python code directly.

Pull Request Checklist

• Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]'
  until you can check this box.
• Unit tests pass (pytest)
• Were system tests modified? If so did you run these at least 5 times to account for the variation across runs?
• System tests pass (pytest test/system)
• Passed code style checking (./style-check.sh)
• You have considered writing a test
• Updated major/minor version in sockeye/__init__.py. Major version bump if this is a backwards incompatible change.
• Updated CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[fhieber]

Thanks, looks good!