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/bootstrap.go

@@ -10,7 +10,7 @@ import (
 	_ "github.com/golang-migrate/migrate/v4/source/file"
 
 	"github.com/italypaleale/francis/components"
-	"github.com/italypaleale/francis/host/local"
+	francishost "github.com/italypaleale/francis/host"
 	"github.com/italypaleale/go-kit/servicerunner"
 	"gorm.io/gorm"
 
@@ -137,7 +137,7 @@ func Bootstrap(ctx context.Context) error {
 }
 
 // actorsRunServiceFn wraps the actor host's Run method in a background service and returns a "ready" signal that other services can wait on
-func actorsRunServiceFn(actors *local.Host) (servicerunner.Service, *servicerunner.Ready) {
+func actorsRunServiceFn(actors francishost.Host) (servicerunner.Service, *servicerunner.Ready) {
 	actorsReady := servicerunner.NewReady()
 	fn := func(ctx context.Context) error {
 		runErrCh := make(chan error, 1)