Basic certificate support

[bigbrett]

Adds basic support for certificate management using wolfSSL Cert Manager.

Client API has add/delete/get functionality for trusted certificate (root), and then a verify function that verifies an (ordered) certificate chain against a given trusted root.

Limitations:

• CM is initialized at verification time to prevent issues managing state between CM and NVM. This may result in slightly more latency for verification operations
• Chain must be ordered
• No support for CRLs (yet)

Future work:

• Feature flag for "minimal" public-key-only verification that doesn't check anything else about certificate validity. Would be useful on space constrained platforms where you really only care about trust and are using X509 to simply interop with existing PKI
• Introduce the concept of a "session" with a live CM instance and directly expose CM via client API so client apps can use it in a more granular way, essentially providing direct (remote) access to the cert manager API from the client
• Better handling of intermediates?

[billphipps]

Looks very reasonable. A few API names and inout questions, but certainly is a good start. I have some concerns about the RAM necessary on the server to parse the certification chain. We may need to preallocate or use some of the existing cache memory to better handle this.

[bigbrett]

Adding this here so we don't forget: Future work should be to refactor error propagation for the verify message to return wolfCrypt error code to client, vs "catch-all" cert verification error