Support providing custom TLS cert

[gjedlicska]

This came up a few times already, where organizations would want to use their own inhouse certificates for the Speckle server.
So far we've diffused these, with its faster and more stable if we manage these and auto renew with Letsencrypt and cert-manager.

But in the case of server instances only accessible on a company internal VPN, we cannot auto generate certificates, cause the server is not accessible on the internet and letsencrypt cannot verify the validity of the certificate request.

[iainsproat]

@gjedlicska - as discussed, I also believe this requires amendment of our nginx configuration: https://nginx.org/en/docs/http/configuring_https_servers.html
We will also need a way of mounting the cert and private key into the container so nginx can retrieve it.

[gjedlicska]

Yeah, this is the route i'd start digging in for docker-compose based scenarios, for k8s setups, we'd need another approach maybe. But let's focus on the compose based setup for now.

[iainsproat]

Closing this in lieu of https://linear.app/speckle/issue/WEB-1633/self-signed-custom-tls-certificate-should-be-supported