Update protobuf mininet p4testgen (#518)

* Support negative-valued action parameters
by converting them in the Python helper code into a positive value
whose bit pattern is the 2's complement representation of the negative
value.

* Update versions of protobuf and grpc installed, install p4testgen
Update protobuf from v3.6.1 to v3.18.1
Update grpc from v1.17.2 to v1.43.2

Both of those versions have been long tested in the p4-guide repo
install scripts.

Update BMv2, PI, p4c, and ptf to latest versions as of 2023-Jun-24

Enable compilation of p4testgen

* Change method to install ptf
I found that the former way that I am replacing did not seem to install
it correctly.

* Make clean.sh remove sources for protobuf and grpc
Saves disk space for the image.  protobuf and grpc source code is easily
obtainable from public sources if the user wishes.

vm-ubuntu-20.04/root-dev-bootstrap.sh

@@ -72,11 +72,6 @@ apt-get install -y --no-install-recommends --fix-missing\
   xcscope-el \
   xterm
 
-# TBD: Should these packages be installed via apt-get ?  They are in
-# my install-p4dev-v4.sh script, but they might not be needed, either.
-
-# zlib1g-dev18
-
 # On a freshly installed Ubuntu 20.04.1 or 18.04.5 system, desktop
 # amd64 minimal installation, the Debian package python3-protobuf is
 # installed.  This is depended upon by another package called
@@ -115,7 +110,7 @@ apt-get install -y --no-install-recommends --fix-missing\
 # At that point, attempting to import any of the 3 modules above gave NO error.
 
 sudo apt-get purge -y python3-protobuf || echo "Failed to remove python3-protobuf, probably because there was no such package installed"
-sudo pip3 install protobuf==3.6.1
+sudo pip3 install protobuf==3.18.1
 
 # Starting in 2019-Nov, Python3 version of Scapy is needed for `cd
 # p4c/build ; make check` to succeed.