Allow disabling role assumption when `AWS_ROLE_ARN` environment variable is set

[JonMerlevede]

Issue Description

The code for dbConnect() checks if the AWS_ROLE_ARN environment variable is set, and performs an explicit assume role operation if it is. To perform the assume role operation, it calls on Boto3. Boto3 also uses AWS_ROLE_ARN to determine which role to assume, for example when using web identity authentication. This causes RAthena to try to assume role A from role A, which is always superfluous and often denied.

Reproducible Example

Set the AWS_ROLE_ARN and AWS_WEB_IDENTITY_TOKEN_FILE environment variables; Boto3 can now authenticate.

Assuming that AWS_ROLE_ARN has value arn:aws:iam::123456789101:role/A, call on dbConnect() and get the exception:

An error occurred (AccessDenied) when calling the AssumeRole operation: User: arn:aws:sts::123456789101:assumed-role/A/botocore-session-xxx is not authorized to perform: sts:AssumeRole on resource: arn:aws:iam::189756888801:role/A

If unsetting AWS_ROLE_ARN, call on dbConnect() and get the exception:

Warning: Error in py_call_impl: botocore.exceptions.InvalidConfigError: The provided profile or the current environment is configured to assume role with web identity but has no role ARN configured.

Proposed solution

Allow a value for the aws_role parameter that does not trigger an assume role operation, even if AWS_ROLE_ARN is set.

[DyfanJones]

Ah sorry about that, i am currently on holiday and won't be able to do any work on this for 2 weeks. However in the meantime I am happy to receive any PRs.