Add cxx support

[subdiox]

Issue #, if available: #166

Description of changes: This PR fixes the build error which occurs when it's built using Swift Package Manager (SPM) with C++ interoperability mode enabled.
Following changes were made:

• AWS_NO_STATIC_IMPL is defined when it's built with C++ complier.
• Extracted enum to the root
• Cast type explicitly if needed
• Avoid using template keyword as a variable name

Dev Test:
I have confirmed that this package could be built successfully with C++ interoperability enabled.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[waahm7]

Thank you for creating the PR. These changes will need to happen in the submodules first. I have created a PR at awslabs/aws-c-common#1160 that solves this problem a bit differently. Once that is approved and merged, I will create PRs for the remaining submodules.

[subdiox]

@waahm7 Thank you so much for quick response. Can I create PRs for other submodules to be followed by myself?

[waahm7]

@subdiox Sure, we will also need a CI job that tests crt-swift with cxx-interop before the final merge.