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/bootstrap/services_bootstrap.go

@@ -5,7 +5,7 @@ import (
 	"fmt"
 	"net/http"
 
-	"github.com/italypaleale/francis/host/local"
+	francishost "github.com/italypaleale/francis/host"
 	"github.com/pocket-id/pocket-id/backend/internal/api"
 	"github.com/pocket-id/pocket-id/backend/internal/apikey"
 	"github.com/pocket-id/pocket-id/backend/internal/appconfig"
@@ -52,7 +52,7 @@ type services struct {
 	emailVerificationModule *emailverification.Module
 	apiModule               *api.Module
 	environmentModule       *environment.Module
-	actors                  *local.Host
+	actors                  francishost.Host
 }
 
 // Initializes all services
@@ -60,7 +60,7 @@ func initServices(
 	ctx context.Context,
 	db *gorm.DB,
 	instanceID string,
-	actors *local.Host,
+	actors francishost.Host,
 	httpClient *http.Client,
 	imageExtensions map[string]string,
 	fileStorage storage.FileStorage,