FR: Add support for via: autogroup:self or similar for routing rules

### What are you trying to do?

Basic scenario:
- Most of our internal users have both a laptop (that changes networks) and dedicated always-on VM ("virtual desktop")
- The always-on VM is in a "trusted" network space
- Both are per-user devices that we wouldn't want to use tags on.

It would be great if users could use their dedicated VM as an exit node for their laptops (or other devices). Or maybe some subnet routing stuff.

### How should we solve this?

Expand via from only supporting tags to also supporting `autogroup:self`, like:
```json
"grants": [
{
"src": ["autogroup:member"],
"dst": ["autogroup:internet"],
"via": ["autogroup:self"],
"ip": ["*"],
},
],
```

### What is the impact of not solving this?

Either can't let users use their own devices as exit nodes, or have to promote per-user devices to tagged server-like devices.

### Anything else?

_No response_