Export FTP server CA cert for client import

Is your feature request related to a problem? Please describe.
Currently the FTP server is using our own self-signed cert when running in SSL mode. There is no way to allow end user clients to import the certificate authority's cert as we don't provide an easy way for them to access it. This makes few FTP clients that allow identifying the cert's CA on connection, e.g. FileZilla.

Describe the solution you'd like
Allow end users to have access to the CA's certificate.

An easiest implementation would be exporting the CA cert to the device's internal storage, then allow end users to copy and import it on their own.

We are not doing this automatically; perhaps a button at FtpServerFragment to export it on demand, plus a Toast or alert dialog prompting the user where to find the CA cert.

Additional context
No direct File I/O please, but SAF even writing to device's internal storage (meaning the MMC for instance, not meaning the app's own internal, protected storage as coined by Android)

Start in FtpServerFragment and trace how the FTP server's self-signed CA certificate is generated or stored. Use Android's Storage Access Framework rather than direct file I/O, and consider how the UI should tell users where the exported certificate was saved. Done means users can export the CA certificate on demand and import it into clients such as FileZilla.