Add option to disable breakpoints

[bkonyi]

We should provide a service RPC to allow for enabling and disabling breakpoints without having to remove and recreate them each time.

VM tracking issue for flutter/devtools#696.

[a-siva]

Why does this have to go in at the service RPC level, can this not be functionality handled by DDS ?

[bkonyi]

This would be simpler to add to the service since we already have a way to enable/disable breakpoints in the VM, it's just functionality that isn't currently exposed.

[a-siva]

I was wondering about whether you would provide the capability of multiple clients to have their own enable/disable state for the breakpoints in which case the VM state would not help.