Implement AWS v4 signature API
Ref T5155. AWS introduced a hugely more-complicated "v4" API at some point, which all (I hope??) services now use.
I implemented it by following this document:
- Implemented all the examples in the document above as unit tests.
- Ran unit tests.
Reviewed By: chad
Maniphest Tasks: T5155
Differential Revision: https://secure.phabricator.com/D14978