Support role chaining in AWS signing filter

*Title*: Support role chaining in AWS signing filter

*Description*:
Currently the AWS signing filter supports a few different ways of getting credentials to use for signing requests. However, as far as I can tell, it does not support role chaining where one set of credentials is used in order to assume another role with which to sign the requests.

In my particular case, I need to configure Envoy as a signing proxy deployed as part of an ECS task, where the identity used to sign requests is not the ECS task role, but another role that the ECS task is able to assume. I would imagine that this would be a new credential provider like `AssumeRoleProvider` that would in turn receive a credential provider chain from which to get the credentials needed to assume the role as needed.

*Relevant Links*:
* [Role chaining documentation](https://docs.aws.amazon.com/sdkref/latest/guide/feature-assume-role-credentials.html)
* [Envoy AWS request signing filter credentials documentation](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/aws_request_signing_filter.html#credentials)