Introduce TLS Settings for go-micro based http services

TLS for the services can be configure by setting the "OCIS_HTTP_TLS_ENABLED",
"OCIS_HTTP_TLS_CERTIFICATE" and "OCIS_HTTP_TLS_KEY" environment variables.
Currently the ocis proxy is this only service that directly accesses backend
services. It determines whether to use TLS or not by looking a the new registry
metadata "use_tls". As specific CA Cert for certificate verification
can be set with the "PROXY_HTTPS_CACERT" environment variable.

services/proxy/pkg/config/config.go

@@ -33,6 +33,7 @@ type Config struct {
 	AutoprovisionAccounts bool            `yaml:"auto_provision_accounts" env:"PROXY_AUTOPROVISION_ACCOUNTS" desc:"Set this to 'true' to automatically provision users that do not yet exist in the users service on-demand upon first sign-in. To use this a write-enabled libregraph user backend needs to be setup an running."`
 	EnableBasicAuth       bool            `yaml:"enable_basic_auth" env:"PROXY_ENABLE_BASIC_AUTH" desc:"Set this to true to enable 'basic authentication' (username/password)."`
 	InsecureBackends      bool            `yaml:"insecure_backends" env:"PROXY_INSECURE_BACKENDS" desc:"Disable TLS certificate validation for all HTTP backend connections."`
+	BackendHTTPSCACert    string          `yaml:"backend_https_cacert" env:"PROXY_HTTPS_CACERT" desc:"The root CA certificate used to validate TLS server certificates of https enabled backend services."`
 	AuthMiddleware        AuthMiddleware  `yaml:"auth_middleware"`
 
 	Context context.Context `yaml:"-" json:"-"`