Add Dynamic thrift union with folly::dynamic serializer

Summary: This adds a thrift_library that declares a Dynamic union that should be pretty easy for folks who want to transport JSON-styled documents over thrift. Additionally, it uses the newly added custom type feature to use a wrapper around folly::dynamic for backing storage in cpp2.

Test Plan: Unit test

Reviewed By: davejwatson@fb.com

Subscribers: alandau, bmatheny, tjackson, akr

FB internal diff: D1541006
Add interface for Thrift Processors in php/hack

Test Plan: build, generate thrift code with server option, inspect Processor class

Reviewed By: davejwatson@fb.com

Subscribers: alandau, bmatheny

FB internal diff: D1542630
Install some header files

Summary:
Proxygen uses some headers that were missing from here. I also
expose the Mocks header files through the install script.

Test Plan: review

Reviewed By: davejwatson@fb.com

Subscribers: doug, alandau, bmatheny

FB internal diff: D1551256
Bump version to 5:0