Improve AWS authentication experience V1

[mikejholly]

As an Earthly user, I find using AWS tools (aws CLI, Terraform, etc.) to be cumbersome. I think Earthly should offer a straightforward and secure way to setup & make use of AWS credentials & help facilitate AWS authentication.

Requirements

• Get the AWS CLI working in Earthly in the simplest way possible
• Do NOT store secrets (in plain text or otherwise) in the layers
• Do NOT rely on any LOCALLY hacks, since they don't work in CI
• Non-interactive when running in CI
• Interactive (e.g. MFA) can be ok locally - debatable?

Implementation

For V1, we can make use of any AWS environmental variables or local configuration. This should include using available OICD credentials configured using GHA steps like aws-actions/configure-aws-credentials.

• ~/.aws directory
• env vars AWS_SECRET_KEY_ID and friends (sample code interacting with and loading from users .aws)
• Some token that OIDC generates (session token?)
  • See how EKS works
  • See also how GHA uses OIDC