Add support for accepting values defined as runtime parameters

[tomasz-baran]

I have a parameter and a task that uses it:

parameters:
  - name: aws_region
    displayName: AWS region
    default: eu-central-1
...
          - task: AWSShellScript@1
            displayName: Get AWS short-term credentials
            inputs:
              inlineScript: |
                echo "Hello"
              regionName: ${{ parameters.aws_region }}
              failOnStandardError: true
              awsCredentials: $(aws_service_connection)
              scriptType: inline

The extension reports the following problem for regionName:

Value is not accepted. Valid values: "af-south-1", "ap-east-1", "ap-northeast-1", "ap-northeast-2", "ap-northeast-3", "ap-south-1", "ap-south-2", "ap-southeast-1", "ap-southeast-2", "ap-southeast-3", "ap-southeast-4", "ap-southeast-5", "ca-central-1", "ca-west-1", "eu-central-1", "eu-central-2", "eu-north-1", "eu-south-1", "eu-south-2", "eu-west-1", "eu-west-2", "eu-west-3", "il-central-1", "me-central-1", "me-south-1", "sa-east-1", "us-east-1", "us-east-2", "us-west-1", "us-west-2", "cn-north-1", "cn-northwest-1", "us-gov-east-1", "us-gov-west-1".

Can the extension use the default value of the parameter to validate the property?

[winstliu]

Going to track this under #420