P4Runtime Release v1.0.0-rc4

Changelog:

• API
  • improved some comments in Protobuf files and fixed comments formatting
  • moved to canonical representation for serializable enums (use integral value, not name)
  • removed complete flag from ReadResponse message
  • improved support for @defaultonly and @tableonly annotations: new scope field for ActionRef message in p4info.proto
  • support architecture-specific notifications on bi-directional P4Runtime stream
• specification
  • fixed typo in error report figure
  • clarified controller role definition
  • fixed value set example
  • added general guidelines on how to pick error codes & specified which error code to return in some common error cases
  • dropped "Lifetime of a Session" section because no one volunteered to write it
  • added link to latest version of PSA specification (v1.1.0)
  • clarified action selector programming and added information about new psa_empty_group_action table property
  • added section giving atomicity requirements for individual write operations
  • added discussion about table size and target requirements for how many entries must be supported
  • updated link to P4_16 specification (v1.1.0)
  • clarified valid values for multicast group ids and clone session ids