digest support in P4 Runtime

[mariobaldi]

Add support for digest from data plane in P4 runtime.

[antoninbas]

Some more context:
digest is an extern in PSA that enables efficient streaming of data from the data-plane to the control-plane (without having to send the whole packet)
PSA implementations are required to support a digest mechanism
The expectation is that P4 Runtime would support opening a stream to the device to retrieve these digests. It seems that this can easily be done using the Read RPC, so nothing would need to change except for the definition of a Digest entity protobuf message.
See the PSA spec for more information on digest: https://p4lang.github.io/p4-spec/docs/PSA.html#sec-packet-digest

Edit: Read may not be appropriate. We may need a bi-directional stream if digests need to be acknowledged by the receiver / client. L2 mac learning example: smac, ingress port tuples are streamed to the client, the client populates the smac / dmac tables, the client acknowledges the tuple to the server. This is at least how bmv2 implements support for digests, in order to avoid flooding the receiver with duplicates.

[antoninbas]

Initial proposal for digest support: #242

[antoninbas]

Support has been added to p4info.proto and p4runtime.proto; I'm closing this issue in favor of #345 (implementation request).