Add support for STS [AssumeRoleWithWebIdentity](https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRoleWithWebIdentity.html).

Includes new config option `iam_web_identity_token` which takes either a WebIdentity token (designed to be passed in with `get_env()`), or a
path to a file containing a WebIdentity token.

Found that in internal tests, only with WebIdentityToken, Terragrunt
fails with:
```
time=2024-06-05T18:11:01Z level=error msg=Error finding AWS credentials (did you set the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables?): NoCredentialProviders: no valid providers in chain. Deprecated.
	For verbose messaging see aws.Config.CredentialsChainVerboseErrors
time=2024-06-05T[18](https://github.com/gruntwork-test/testing-terragrunt-with-web-identity/actions/runs/9389092410/job/25855946545#step:6:19):11:01Z level=error msg=Unable to determine underlying exit code, so Terragrunt will exit with error code 1
```

Fixed by updating AssumeIamRole