Update M1 and M3 interfaces to return the number of purged entries for the cache purge operation

[davidjwbbc]

In order for the 5GMS AF to inform the 5GMS Application Provider about the number of cache entries that were purged during a purge request at referencing point M1:

• The 5GMS AS needs to inform the 5GMS AF about the number of cache entries that were purged during a purge request at reference point M3.

To achieve this objective, two distinct success status codes need to be generated at these reference points:

• 204 (No Content) if the request was valid but no cache entries were purged (none matched the regular expression in the request),
• 200 (OK) if one or more cache entries were purged, with the message body being an integer number of cache entries purged.

[rjb1000]

Looks good. Thanks, @davidjwbbc.