Separate PKCS#11 Code

[skemper-sd]

Issue #, if available: #86

Description of changes:

This PR is a first whack at separating out the code that links to native PKCS#11 libraries on the hosts. The goal is to build a version of aws_signing_helper that doesn't link to the library and so can be compiled without CGO. It loses PKCS#11 support in the process, but that's an acceptable loss for some users.

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[skemper-sd]

There are (at least) a few obvious questions:

1. It makes more sense to me to view the PKCS#11 support as an optional module, so the build script is structured that way. However, I don't see your build scripts themselves in this repo, so I can't set them up to include the new envvar.
2. I haven't (yet) tested any of this code in real life, I just opened the draft PR to start the discussion about the approach. Using build tags might not be the best way to do it! make test works though.