Fix handling of {:system, "AWS_ROLE_ARN"}

[mbuffa]

Hello there,

Fixes #43

Also:

• Updates Test/Dev dpeendencies (test suite wasn't running on Elixir 1.18)
• Fixes the existing three failing tests (were missing proper mocks)
• Fixes one deprecation
• Addd a test to check that we correctly source AWS_ROLE_ARN from the system

[mbuffa]

Closing for rework

[mbuffa]

Ok. This should be better now.

Previously, we were calling auth.role_arn in get_security_credentials, so this function always expect it, so I ensured the upper call checks for role_arn presence.

Also simplified the code to make ExAws.Config.new load the Role ARN, and added a test to make sure the lib still works if we omit the role_arn arg.