Add a withAWS plugin (as alternative to Awssume)

[kmanning]

The awssume plugin requires installing awssume on your slave.

An alternative plugin based on using the role assumption feature of withAWS would remove that dependency.

[kmanning]

UseCase 1(a): Assume environment-specific roles

WithAwsPlugin.withRole().init()
...
def testAndPackage = new TerraformValidateStage()
// Execute in an withAws block, using `role` of AWS_ROLE_ARN, or QA_AWS_ROLE_ARN
def deployQa = new TerraformEnvironmentStage('qa')
// Execute in an withAws block, using `role` of AWS_ROLE_ARN, or UAT_AWS_ROLE_ARN
def deployUat = new TerraformEnvironmentStage('uat')
// Execute in an withAws block, using `role` of AWS_ROLE_ARN, or PROD_AWS_ROLE_ARN
def deployProd = new TerraformEnvironmentStage('prod')

UseCase 1(b): Explicitly define role

WithAwsPlugin.withRole('someRoleArn').init()
...
def testAndPackage = new TerraformValidateStage()
// Execute in an withAws block, using `role` of `someRoleArn`
def deployQa = new TerraformEnvironmentStage('qa')
// Execute in an withAws block, using `role` of `someRoleArn`
def deployUat = new TerraformEnvironmentStage('uat')
// Execute in an withAws block, using `role` of `someRoleArn`
def deployProd = new TerraformEnvironmentStage('prod')

[kmanning]

Did not implement withCredentials. That should be a separate Issue.

Implemented with standard global variable hierarchy. That should be the preferred method.