Remove dependency on phpseclib

Problem/Motivation

@freefish has a dependency conflict cited in #3269077: How to downgrade to become compatible with phpseclib 2? → . phpseclib is a useful library, but the requirements we have in this module are simpler and can be achieved with the PHP OpenSSL functions.

Proposed resolution

Refactor the code in the KeyPair class to replace the phpseclib calls with OpenSSL calls.

API changes

Ideally, none. The KeyPair class should behave in the same way, excluding performance metrics.