Fix Fargate Agent not handling int provided cpu and memory

[joshmeek]

Summary

Starting the fargate agent through the CLI could lead to issues in parsing of the cpu and memory values. This is due to the fact that boto3 required these values to be strings however natively they appear as integers. e.g.:

["prefect", "agent", "start", "fargate", "cpu=256", "memory=1024"]

Normally we exclude the literal parsing of these values because they are expected to be strings however we need to special case when they are actually provided as integers.

Importance

This fix will unblock some users attempting to do this however the new RunConfig pattern will completely resolve and simplify all of the Fargate agent parsing so much.

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

[codecov]

Codecov Report

Merging #3423 into master will increase coverage by 0.01%.
The diff coverage is 66.66%.