Add configuration to trust a selfsigned Certificate Authority / Certificate

Since 2023.1, embedded provider uses the external authentik domain to access authentik. If the HTTPS certificate used by authentik is self signed, it might break the authentication and redirection process.

**Describe alternatives you've considered**
The only solutions currently available to bypass this issue are:
- Manually rebuild the docker images with an updated /etc/ssl/certs/ca-certificates.crt (Quite heavy process)
- Enable authentik_host_insecure (Does not seems to work currently and might be quite insecure)
- Use HTTP instead of HTTPS (Insecure)

**Describe the solution you'd like**
A more user friendly and secure solution than authentik_host_insecure might be to add a configuration option to trust a CA Certificate stored in System -> Certificates.

Thank you :)