Allow setting maximums on public services consumption

[jdknives]

Feature description

Both the VPN-server registration and the public visor registration are useful features for the network but do not currently allow specifying limits on their respective resource consumption on the host. This potentially leads to resource exhausiton of the host and makes it more difficult and potentially risky in terms of resource consumption for owners of Skywire nodes.

We need to allow operators to specify bandwidth, computing and memory resource limits when running in these modes.

[0pcom]

Required for this:

• config file fields for specifying these limits
• config gen flags for setting these limits
• a mechanism for enforcing these limits in the source code