Add aws_session_token kwarg to FargateTaskEnvironment

[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?

Adds the option to specify an aws_session_token for the FargateTaskEnvironment

Why is this PR important?

This is useful when in an IAM / ARN setup

[codecov]

Codecov Report

Merging #1688 into master will increase coverage by <.01%.
The diff coverage is 100%.