Expand file-based storage to all other storage types

[joshmeek]

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What does this PR change?

This PR closes #2887 by expanding file-based storage (and hot reloading flows) to all other current storage types. This adds a stored_as_file: bool kwarg to the storage types that also support pickle storage.

Example Workflow

# flow.py
from prefect import task, Flow
from prefect.environments.storage import S3

@task
def imi():
    print("I am, I am, I am, I am")

f = Flow("s3-file", tasks=[imi])
f.storage = S3(bucket="my-flows", key="flow.py", stored_as_file=True)

Upload to S3:

aws s3 cp flow.py s3://my-flows/flow.py

Register with backend:

prefect register flow -f flow.py -p "Demo"

Opening this PR to get some feedback on the implementation. I plan on adding some more documentation around this pattern and making the storage documentation better. I also need to revamp the healthcheck.py for Docker storage because it is pickle-based right now (current implementation skips over healthcheck if using a file but will re-add it once working 👍)

[codecov]

Codecov Report

Merging #2944 into master will decrease coverage by 0.00%.
The diff coverage is 90.78%.

[jcrist]

Woot, this is great to see!

This adds a stored_as_file: bool kwarg to the storage types that also support pickle storage.

Since pickles are also stored as files (it's just the type of the contents of the file that is different), I'm not sure if that's the best kwarg. Perhaps something with py or python or script in the name?

[jcrist]

LGTM!