Undefined symbol errors when consuming the library as shared/dynamic

The `"-fvisibility=hidden"` (see [CMakeLists.txt#L77](https://github.com/awslabs/aws-lambda-cpp/blob/master/CMakeLists.txt#L77)) in `CMakeLists.txt` causes undefined symbols errors when using this library as shared object.

It's not a problem when using it as a static library because visibility settings doesn't affect that by design.

There are multiple solutions for this problem:
1. Just remove `"-fvisibility=hidden"`. We are currently using this approach as a workaround because we currently don't have many symbols to be considered private, this may be a viable option.
2. Keep `"-fvisibility=hidden"` and manually annotate symbols intended to be public with: `__attribute__((visibility("default")))`, however, this will require a significant amount of effort.
3. Replace `"-fvisibility=hidden"` with `"-fvisibility=default"` and manually annotate symbols intended to be private with `__attribute__((visibility("hidden")))`

Start with the visibility setting at CMakeLists.txt#L77 and reproduce the undefined-symbol errors when consuming the library as a shared object. Compare the listed visibility approaches and confirm that the chosen change preserves static-library use while exposing the symbols required by shared-library consumers.

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