[Core][AWS] Allow assumption of AWS Credentials provided in ECS Containers

[JGSweets]

Prior to this PR, ECS containers could not launch skypilot jobs because as the type is considered: container-role as opposed to iam-role when evaluating aws configure list.

This PR:

• Allows ECS container assumed iam credentials to be found by adding the CONTAINER_ROLE value to the AWSIdentityType.
• adds .env to the .gitignore

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Evaluated sky check w/o the update in an ECS container (failed)
  • Evaluated running sky up ... w/o the update in an ECS container (failed)
  • Evaluated sky check w/ the update in an ECS container (passed)
  • Evaluated running sky up ... w/ the update in an ECS container (passed)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

Thanks for adding the support for container_role @JGSweets! This looks great to me.