Bug 1900132 - streamline PVerifySSLServerCert protocol r=jschanck

Previously the PVerifySSLServerCert protocol consisted of two functions: one to
call when certificate verification succeeded, and another to call upon failure.
This was unnecessary, as the code before and after this protocol didn't have
the same split. This patch unifies the protocol to better match the surrounding
code. It also takes the opportunity to make use of some IPC helpers to
serialize enums rather than manually casting to and from basic integer types.

Differential Revision: https://phabricator.services.mozilla.com/D212594