Use symlink for consistent path to unstable/ headers

[graebm]

Issue:

External projects can always do:
#include <s2n.h>

But, for the unstable headers the path is either
#include <s2n/unstable/cleanup.h>
or
#include <unstable/cleanup.h>

...depending on whether s2n was installed before your project, or built at the same time as your project via CMake's add_subdirectory(s2n). CRT language bindings like aws-crt-python build s2n via add_subdirectory.

Description of changes:

Add symlink, from api/s2n -> api/unstable.

This way, external code can always do #include <s2n/unstable/cleanup.h>, and it works whether s2n was pre-installed, or is being built simultaneously.

This symlink is not installed. It only affects builds that use s2n's source code directly.

Call-outs:

An alternate fix is here: #4825
This simply moves the files, instead of relying on symlinks, and fixes up all local code to use the new paths. It's a lot more changes, but feels to me like a simpler long term fix.

I'm not sure how 100% portable symlinks are. For example, zip requires a --symlink flag to handle them properly.

Testing:

• Ensured unstable headers still install to same location
• Ensured code still built when using old unstable/*.h paths
• Ensured code built when using new s2n/unstable/*.h paths

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[graebm]

symlinks are too weird to rely on