FR: Allow routes to be exposed to shared devices

### What are you trying to do?

I am requesting a feature to be added that would allow users to choose whether to share routes advertised by a device when it is shared with other users.

### How should we solve this?

By creating an option to allow advertised routes to be shared when you share a device. You can have the default set to no, but the option to allow it with a quick ? explanation would be great.

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

2+ users cannot have access to the same node and be able to use the routes that node is advertising. So, 1 user gets to own the device and have access to the advertised routes and the other users have to use an alternative solution. In my case that would be an IPSEC tunnel. We are trying to move away from IPSEC as much as possible and would love to use Tailscale exclusively for our site to site needs. The devices we are using are a mixture of pfSense and OPNsense.

### Anything else?

Apologies if this has already been requested. I didn't see it anywhere in my searching. Having the option in the access controls section would be one way, but if it could just be a simple toggle in the device sharing window, like the exit node toggle, that would just make it more user friendly and accessible to more folks. Thanks for your consideration.