Add custom clang attributes

[brklyn8900]

Deliverable: have a proof of concept that decorates function names with [[koinos::action]] and autogens the dispatcher

[sgerbino]

Some helpful links:

https://meet.google.com/linkredirect?authuser=0&dest=http%3A%2F%2Fclang.llvm.org%2Fdocs%2FInternalsManual.html%23how-to-add-an-attribute

https://meet.google.com/linkredirect?authuser=0&dest=https%3A%2F%2Fgithub.com%2FEOSIO%2Fllvm%2Fblob%2F3c026f8b7813ad693749d3551353a554c5ca33e7%2Flib%2FTransforms%2FEosioApply%2FEosioApply.cpp