Fargate Task Environment

[joshmeek]

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

Adds the FargateTaskEnvironment
Closes #1549

Why is this PR important?

Works in a similar way to the KubernetesJobEnvironment by allowing users to specify completely custom task definitions for their flow runs

[codecov]

Codecov Report

Merging #1592 into master will increase coverage by <.01%.
The diff coverage is 95.31%.

[cicdw]

A few minor typos and a question - nicely done!