prototype of packet drops in p4 w/ possibility

[skyWalkerZJ]

I write a simple P4 code and a simple introduction about packet drop. I test it in tofino model and get OK. Maybe there are still some problems about this task. i will finish the test scripts in the next days.

[r12f]

woot! great! I left a few comments and let's see if we could add them in as well. and so far the poc progress looks great!

[r12f]

#7