Pass a role ARN in runtime and use with service connection

**Is your feature request related to a problem? Please describe.**

Assume that I have an IAM user for Azure DevOps and the IAM user can switch two roles in a dev account and a production account.
I want to use one service connection for the IAM user and pass a role ARN in build runtime.

However `AWSShellScript@1` task (and other tasks) don't accept a role ARN as input paramter. Now, I manually AssumeRole in `AWSShellScript@1` task and set variables like `AWS.AccessKeyID` as below.

```yaml
jobs:
- job: job_in_dev
variables:
AWS.Region: ap-northeast-1
roleArn: arn:aws:iam::123456789012:role/azure-devops-DevRole
sessionName: $(Build.BuildId)
steps:
- task: AWSShellScript@1
displayName: AssumeRole
inputs:
awsCredentials: aws-sample-connection
scriptType: inline
inlineScript: |
aws sts assume-role --role-arn $(roleArn) --role-session-name $(sessionName) \
--query 'Credentials.[AccessKeyId, SecretAccessKey, SessionToken]' --out text \
| while read -r v1 v2 v3; do
echo "##vso[task.setvariable variable=AWS.AccessKeyID;issecret=true]$v1"
echo "##vso[task.setvariable variable=AWS.SecretAccessKey;issecret=true]$v2"
echo "##vso[task.setvariable variable=AWS.SessionToken;issecret=true]$v3"
done
- task: AWSShellScript@1
displayName: Do the job in dev account
inputs:
scriptType: inline
inlineScript: aws ec2 describe-instances
```

This method works but the first step is too redundant.

**Describe the solution you'd like**

I suggest to add a task to AssumeRole (ex: `AWSAssumeRole@0`). If `AWSAssumeRole@0` accept a service connection (`awsCredentials`) and a role ARN, the above yaml is simplified.

```yaml
jobs:
- job: job_in_dev
variables:
AWS.Region: ap-northeast-1
steps:
- task: AWSAssumeRole@0
inputs:
awsCredentials: aws-sample-connection
assumeRoleArn: arn:aws:iam::123456789012:role/azure-devops-DevRole
roleSessionName: $(Build.BuildId)
externalId: xxx
# subsequent steps can use a shot-live credential genereated by AssumeRole
- task: AWSShellScript@1
displayName: Do the job in dev account
inputs:
scriptType: inline
inlineScript: aws ec2 describe-instances
```

**Describe alternatives you've considered**

An alternative solution is a method to pass a role ARN as build variable. If `AWSShellScript@1` (and other tasks) read a role ARN from build variable like `AWS.AssumeRoleArn`, the above yaml is simplified.

```yaml
jobs:
- job: job_in_dev
variables:
AWS.Region: ap-northeast-1
AWS.AssumeRoleArn: arn:aws:iam::123456789012:role/azure-devops-DevRole
AWS.RoleSessionName: $(Build.BuildId)
AWS.ExternalId: xxx
steps:
- task: AWSShellScript@1
displayName: Do the job in dev account
inputs:
awsCredentials: aws-sample-connection
scriptType: inline
inlineScript: aws ec2 describe-instances
```

This solution is already suggested by @FaridNeshat-TomTom in https://github.com/aws/aws-toolkit-azure-devops/issues/340#issuecomment-805734538.

The issue describes adding runtime role-ARN support to Azure DevOps AWS tasks, either through an AWSAssumeRole task or shared build variables. Start by locating the existing task and service-connection implementations, then compare both proposed approaches and their credential-handling implications. Done means the selected approach supports the shown YAML flow and is covered by appropriate tests.

Written by the indexing model from the issue text.