feat: add FRANCIS_HOST to connect to a standalone Francis runtime

FRANCIS_HOST decides where the Francis actor runtime lives. When set to "embedded" (the default), Pocket ID starts the runtime inside its own process.

Any other value is the address, or a comma-separated list of addresses, of a standalone Francis runtime. Pocket ID then connects to it as a remote actor host and starts no embedded runtime.

Because when using a remote runtime, it's likewise not possible to enforce a single instance of Pocket ID is running at once, the env vars currently have the `EXPERIMENTAL_` prefix, are **undocumented**, and show a warning if used.

Notes:

- Connecting to a standalone runtime also needs FRANCIS_HOST_PSK or FRANCIS_HOST_JWT_FILE, and optionally (but recommended) FRANCIS_CA.
- When connecting to a remote runtime, exporting Pocket ID data does not include the actor state, which will need to be backed up and restored separately

backend/internal/ldapsync/module.go

@@ -7,7 +7,7 @@ import (
 	"net/http"
 
 	"github.com/gin-gonic/gin"
-	"github.com/italypaleale/francis/host/local"
+	francishost "github.com/italypaleale/francis/host"
 	"gorm.io/gorm"
 
 	"github.com/pocket-id/pocket-id/backend/internal/appconfig"
@@ -43,7 +43,7 @@ type ScimSyncScheduler interface {
 
 type Dependencies struct {
 	DB          *gorm.DB
-	Actors      *local.Host
+	Actors      francishost.Host
 	HTTPClient  *http.Client
 	FileStorage storage.FileStorage