Support exporting the Proxyman CA Certificate in binary (DER) format

The ability to export the CA cert directly into a binary format would be a great benefit to devs that are proxying AppleTV hardware.

Being able to simplify the configuration (even by 1 step) would be beneficial as the Apple Configurator does not accept PEM-based certs and making it appear that Proxyman doesn't work with AppleTV hardware. Exporting directly into a binary format would make the process for proxying AppleTV hardware identical to that of Charles Proxy and allow developers who are migrating from Charles to Proxyman to make the move as seamless as possible.

Proxyman version? (Ex. Proxyman 1.4.3)

2.24.0

macOS Version? (Ex. mac 10.14)

11.3

Steps to reproduce

1. Certificate -> Export Proxyman CA Certificate
2. Notice that the format of the Save As field is prepopulated as proxyman-ca.pem with no additional options available.

Expected behavior

Have the ability to export in a binary format.

Screenshots (optional)