Add ability to load IdP and SP certs from filesystem

[astro-stan]

Summary

I want to be able to select certificates to be used for IdP and SP from a directory on the host.

For example, have a /certs directory where Nextcloud will look for certs and allow the user to choose which to use.

Use case

I am hosting nextcloud as part of a kubernetes cluster, I need this so I can mount a certs directory into the Nextcloud container, set Nextcloud to use them, and periodically replace the certs when they expire, without having to reconfigure the SAML settings in Nextcloud.