Add Any protobuf message to CapabilitiesResponse for experimental features

[verios-google]

The Any protobuf message would allow parties to experiment with new capability responses before proposing them for standardization. This provides a convenient way to try experimental features while remaining in sync with the repo.

A quick guide for how to use the Any message can be found here

An example of how this may be used:

// A user would like to experiment with a new capability, so they define a proto
message ToasterCapabilities {
  int max_bread = 1;
  bool water_resistant = 2;
}


// The hardware (a toaster in this case) can then populate the response:
ToasterCapabilites toaster;
toaster.set_max_bread(2);
toaster.set_water_resistant(false);
CapabilitiesResponse response;
response.mutable_experimental->PackFrom(toaster);


// The consumer (typically a controller) can then use the info:
ToasterCapabilites toaster;
response.experimental.UnpackTo(toaster);
if (toaster.max_bread < 10)
  std::cout << "This toaster sucks";

This example is for fun but one could imagine a plethora of capabilities a switch/NIC/hardware may advertise. The P4 runtime specification does advertise some capabilities (e.g. table sizes) and the p4 constraints repo allows for more complex restrictions on entries, but there is still a large number of capabilities that cannot be captured by current solutions (e.g. two tables sharing resources).

NOTE: In the future, it would super nice to migrate p4runtime.proto from proto3 to proto editions. When that happens, this (and possibly other existing) Any protobuf message(s) should be replaced with proto extensions. I added a comment in the spec for awareness.

[chrispsommers]

Looks interesting, let's discuss at the next P4-APi WG meeting!

[chrispsommers]

LGTM

[verios-google]

@chrispsommers can you merge this, I'm not authorized to do this. Unless you're waiting on someone else's approval

[smolkaj]

A few nits

[jonathan-dilorenzo]

Thanks Victor. Basically just what Steffen said.

[jonathan-dilorenzo]

LGTM

[smolkaj]

Final nit, approving early.

[smolkaj]

Chris already approved, so I'll go ahead and submit.