Blocking shutdown option

• Adds an option to the ApiHandle to allow destruction to wait for all thread-based CRT objects to terminate before completing.

Update s2n to release v0.10.19

• Update s2n to release v0.10.19

Reduced submodule path length to help on windows

• BUILD FIX: Reduced submodule path length to help on windows. From "aws-common-runtime" -> "crt".
• FEATURE: Bring back Aws::Crt::StringView and Aws::Crt::ImdsClient, which were temporarily removed in the previous release.

Build fixes

Build fixes by updating CRT to v0.10.5

• BUILD FIX: Temporarily remove Aws::Crt::StringView and Aws::Crt::ImdsClient until cross-platform build issues are figured out.
• BUILD FIX: Fix aarch64 detection/support.
• BUILD FIX: Update S2N to solve the compiler warning.

Improved timeouts

Update aws-crt-cpp to v0.10.3, which brings:

• BUGFIX: Many objects with async shutdown now use ref-counting under the hood. Should greatly reduce use-after-free errors. This does not affect the C++ API.
• BUGFIX: Increase default TLS-negotiation timeout from 4s to 10s.
• BUGFIX: Ensure bootstrap callback data is allocated by the bootstrap's allocator
• FEATURE: Instance Metadata Service (IMDS v2) Client
• FEATURE: Expose MqttConnection::SetReconnectTimeout(min, max)

Upgrading CRT version

• API CHANGE: awscrt.auth.AwsSigningConfig.signed_body_value is now a string instead of an enum.
• API CHANGE: Added check for iOS platform
• BUGFIX: Fixes possible crash If the underlying http_message get destroyed before the destructor of HttpMessage.
• BUGFIX: Fixes a crash when shutting down an mqtt connection with an incomplete request with no callback from aws-c-mqtt
• BUGFIX: Crash fix caused by passing null into a shared_ptr reset

CMake Config Fixes

• Contains a fix for an incorrectly templated submodule (Aws-checksums) cmake config that would break consumption of the SDK via cmake.

Websocket close time out task

During the shutdown process, websocket will wait for the close frame to be writen. However, it's possible that some message blocks the io, and we need a timeout task to cancel the close frame.

fix race condition & possible momery leak from dependency

Readme (#126)

standardized README format
moved examples to sample folder README

X509 support and sample

• Adds support for the x509 credentials provider
• Adds command line options for x509 credentials sourcing to the pub sub sample