Use cached boto3 clients in ECSWorker

[kevingrismore]

Closes PrefectHQ/prefect#7805

Replaces the use of a session object for client creation with a single cached client per client type in the ECS worker. Significantly reduces memory usage when handling multiple flow runs.

Screenshots

Relative memory consumption (depicted by width of allocating call stack) during 50 simultaneous deployment runs, before and after:

Peak memory usage: 902.5 MiB

Peak memory usage: 45.5 MiB

Checklist

• References any related issue by including "Closes #" or "Closes ".
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[urimandujano]

Nice work, the improvements in your screenshot are incredible!

[desertaxle]

We recently added client caching for AwsCredentials in #369. Does that solve the issue or do we need additional caching at the ECSWorker layer?

[kevingrismore]

We recently added client caching for AwsCredentials in #369. Does that solve the issue or do we need additional caching at the ECSWorker layer?

This change just takes advantage of the caching from AwsCredentials as previously the worker was grabbing a session from the credentials and building multiple clients via one of the session's methods.

This only wouldn't be advantageous if someone was running lots of deployments, and each of those deployments had unique AwsCredentials.