Release 1.12.0 of bmv2

List of new features:

bmv2 core:

• use uint32_t consistently for ports
• support for saturating casts
• new macros to easily register floating C++ functions as extern functions which can be referenced in the JSON

simple_switch:

• new command line option --load-modules to dynamically load object files (which may include extern definitions)

simple_switch_grpc:

• support latest PI / P4Runtime; in particular support new P4Runtime package names (which include the major semantic version number)
• support for PI multicast functions
• new id field for PacketStream to identify packets sent to & from simple_switch_grpc on the dataplane gRPC stream (not to be confused with the P4Runtime packet IO stream)
• better README
• respect exact P4Runtime definition of "priority" (for ternary matches) in implementation: 0 is not a valid priority and a higher priority value means a higher logical priority

psa_switch:

• skeleton for PSA implementation, not really usable at this stage (let's target this for 1.13)