PSA Digest extern small notes

[jafingerhut]

Consider adding a note that an optional enhancement would be to maintain 'drop counts' for each Digest extern instance, that counted how many times a P4 program called the method to send a digest, but there was no room to store it, because driver and/or other software was not draining the memory used to store digests quickly enough.

Perhaps the second idea here would be best as a future PSA addition, but having the option in a Digest extern instance to eliminate duplicates would be useful, e.g. if it was a digest used for indicating that a source MAC address was not present in a MAC table, there is often no need to send duplicate messages to the control plane if multiple packets arriving near in time to each other send the same data in the digest.

[jafingerhut]

At least the 'drop count' portion of this issue is addressed in this PR: #558

[jafingerhut]

Leaving this issue open as a possible future enhancement to the PSA spec: the idea of standardizing an option for a Digest that makes it 'de-duplicating'. This is something that some switch ASICs do today for MAC learn notifications, for example -- if a burst of them occur near the same time, the hardware and/or driver software avoid storing duplicates.