Support PKCS#11 for mutual TLS on Unix platforms

Support PKCS#11 for mutual TLS on Unix platforms (#451)

Merging the `pkcs11` feature branch to `main`. Previous Pull Requests can be found here:

- https://github.com/awslabs/aws-c-io/pull/410 - Add PKCS#11 headers
- https://github.com/awslabs/aws-c-io/pull/408 - Public API first pass
- https://github.com/awslabs/aws-c-io/pull/412 - Implement library load/unload
- https://github.com/awslabs/aws-c-io/pull/413 - Get tests running in CI
- https://github.com/awslabs/aws-c-io/pull/425 - Find private key / begin s2n integration
- https://github.com/awslabs/aws-c-io/pull/428 - Finish integration with s2n
- https://github.com/awslabs/aws-c-io/pull/430 - Add tests. Each test now sets up its own tokendir.
- https://github.com/awslabs/aws-c-io/pull/431 - Misc fixes
- https://github.com/awslabs/aws-c-io/pull/432 - Add TLS test
- https://github.com/awslabs/aws-c-io/pull/434 - Handle connection failure during PKCS#11 operations
- https://github.com/awslabs/aws-c-io/pull/439 - Support multiple digest types for RSA
- https://github.com/awslabs/aws-c-io/pull/440 - Misc fixes
- https://github.com/awslabs/aws-c-io/pull/445 - Each CKR_ return value has its own AWS error-code
- https://github.com/awslabs/aws-c-io/pull/443 - Add license for PKCS#11 headers
- https://github.com/awslabs/aws-c-io/pull/442 - Behavior enum controls how C_Initialize() and C_Finalize() are called.
- https://github.com/awslabs/aws-c-io/pull/450 - Misc fixes

Co-authored-by: Prateek Yadav <63616065+prateek-y@users.noreply.github.com>