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/common/testutils_unit.go

@@ -0,0 +1,10 @@
+//go:build unit
+
+// This file contains utils for unit tests and it's only built when the "unit" tag is set
+
+package common
+
+// SetFrancisAddresses sets a value for francisAddresses
+func (c *EnvConfigSchema) SetFrancisAddresses(v []string) {
+	c.francisAddresses = v
+}