Add public key pinning

[DBL2017]

Added a configuration item called “publicKey” to represent the server’s public key, which is used for pinning the server certificate with a public key.

[fpagliughi]

Actually, this might be nice to have.

But, unfortunately, one thing you missed is how the library handles modifications to the public data structures, particularly the options like MQTTAsync_SSLOptions.

Basically, you should preserve the pre-existing binary layout; so only add new elements to the end of the struct. Then update the structure version number, and only access the new elements in the code if the in-memory struct has a version greater-than or equal to the version where you added the new elements.

But be careful when doing this. The current version number of the structs is hard-coded at a few places in the code. This should probably be fixed with some #define constants, but at the moment this is the way.

There's a similar discussion of this in another pending PR:
#1463 (comment)