File-based flow storage

[joshmeek]

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• adds new tests (if appropriate)
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What does this PR change?

Closes #2785

This is a first cut of an implementation of file based storage. There are a few new additions in this PR:

• GitHub storage which references flows stored as .py files in github repos
• A storage utility for extracting flow objects from .py files
• prefect register flow CLI command which takes a .py file and registers a flow with a backend

As envisioned in this PR, the typical workflow for file based storage might look like:

Compose flow .py file where flow has GitHub storage:

flow = Flow("my-flow")
flow.storage = GitHub(repo="my/repo", path="/flows/flow.py")

Push this flow.py file to the my/repo repository under /flows/flow.py.

Call prefect register -f flow.py to register this flow with GitHub storage.

Now as long as the flow structure is the same it does not need to be reregistered if the content of the flow itself changes. All that needs to happen is the flow file in the set location is overwritten (e.g. pushed to the git repo). This means flows may now be updated easily in CI/CD processes, development, mid run, between runs, etc...

A couple extra TODOs:

• Gate register, run, etc.. with a flag during run in order to prevent rerunning if found in flow file
• Evaluate labeling
• Resolve GitHub storage one-to-many challenge

Why is this PR important?

Opens up the door for new file based storage, flow cloning, and "hot reloading" flows.

[codecov]

Codecov Report

Merging #2840 into master will decrease coverage by 0.03%.
The diff coverage is 88.34%.

[cicdw]

a few questions - exciting stuff!

[cicdw]

minor doc tweak, otherwise LGTM!