Nested tls support

• Added support for nested tls when using a proxy

Try-Lock fix on Windows

• Fixes a fatal assert on Windows when try-lock was called

UnifiedAgent supporting code added

• UnifiedAgent supporting code added
• Builder configuration updated
• Main ranch references updated

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.