libcrypto types could be put into a .h

Currently, many of them are in the .c, which makes them harder to reference.

_Originally posted by @danielsn in https://github.com/aws/aws-encryption-sdk-c/pull/582#discussion_r482348527_

Start by locating the libcrypto type definitions that are currently in C source files and identify which header should expose them. Move the appropriate types so they can be referenced outside those source files, then verify that existing references still build and the project tests pass.

Written by the indexing model from the issue text.