Flow save / load

[cicdw]

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• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
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What does this PR change?

This PR introduces a save / load interface to Flows for a smoother deployment experience.

Why is this PR important?

Some users were annoyed with cloudpickle's behavior if you create a Flow in a file called "my_flow.py" and then imported that flow inside "my_deploy.py" like from my_flow import flow, deploy health checks would fail. This is because cloudpickle sees your flow's logic as living inside a module called "my_flow".

Using the new save / load interface allows users to save their flow at build time, so that cloudpickle correctly manages the imports.

[codecov]

Codecov Report

Merging #1685 into master will increase coverage by <.01%.
The diff coverage is 100%.

[aroder]

Yeah! This is helpful. I find that creating a "build" folder at the top level of my project and excluding it from source control is a convenient place to put the pickled flow.