feat: configure ExAws without env vars

[thecristen]

In the interest of best security practice, we can remove reliance on hardcoded AWS_ACCESS_KEY_ID and
AWS_SECRET_ACCESS_KEY environment variables by configuring the ExAws dependency in this particular way! I also did some light updating to associated docs, which includes using AWS CLI.

Can the current devs confirm (via approval or comment on this PR) that this setup works on their machines? @kotva006 @anthonyshull @amaisano

[anthonyshull]

My concern is that this won't work running locally in Docker. My preference would be to create a mock of ExAws and use that locally. Then we could get rid of this configuration altogether.

[kotva006]

Works pretty slick

[anthonyshull]

This assumes that the configuration being used is the default configuration. This won't work for people for whom these credentials are under another configuration name.

[thecristen]

Good call, I'll switch it to using the system's via (from ExAws):

Alternatively, if you already have a profile name set in the AWS_PROFILE environment variable, you can use that with {:awscli, :system, timeout}

[anthonyshull]

@thecristen pretty sure we don't need this now that we have this working both locally and in docker compose

[thecristen]

Covered elsewhere.