Make cryptography dependency optional

@matthew.s1 posted this comment here:

Hey, not specific to this change but we have been trying to incorporate the sial2 package into our repo but have been encountering some issues. The main issue is that they depend on cryptography which doesn’t provide prebuilt arm v7 binaries, which means we need to build it ourselves and it makes our build pipeline extremely slow (4-5x slower) and will require us to do significant amount of messy optimizations to keep our image size to a similar magnitude. I understand this is needed for your custom CA handling, but if you could just make it optional somehow (Python dependency management is very messy), then it would be enough - Thanks!