Add initial entries to P4 tables, in addition to current const entries

[jafingerhut]

• Define "initial entries" for P4 tables, in addition to currently defined "const entries" p4-spec#1159

Status as of 2024-Sep-14: Changes have been made in the language spec in 2023, and the p4c compiler implements writing of a new has_intitial_entries property for tables, and a new const Boolean property for entries in the optionally written entries output file.

Still not done yet: Implementation of initial entries in the BMv2 software switch simple_switch_grpc. See this README and the corresponding test P4 programs and Python PTF tests in this directory:

• https://github.com/jafingerhut/p4-guide/tree/master/initial-table-entries