aws: assert in debug for async credential providers

[nbaws]

If you are reporting any crash or any potential security issue, do not
open an issue in this repo. Please report the issue via emailing
envoy-security@googlegroups.com where the issue will be triaged appropriately.

Title: Assertion failure in debug build due to accessing timer outside of main thread

Description:
Tracking issue for fixing assertion failure in AWS request signing extension.

Repro steps:
Using aws-request-signing-filter.yaml and enabling runtime guarded feature envoy.reloadable_features.use_http_client_to_fetch_aws_credentials, then sending a request to envoy.

[phlax]

cc @derekargueta @suniltheta @mattklein123 @marcomagdy

[nbaws]

pr coming to address this issue