Adjust Fargate Agent defaults and env var parsing

[joshmeek]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Updates the Fargate Agent boto3 access kwargs to default to None
Properly parses env vars that may be in forms such as lists and dicts

Why is this PR important?

Not providing certain access kwargs was failing on initialization of the boto3 client and not properly parsing certain env vars was rendering them useless

[codecov]

Codecov Report

Merging #1687 into master will increase coverage by <.01%.
The diff coverage is 100%.