Add AEP-8 - AEP Style and Guidance

[mkistler]

This is a fresh attempt at adding AEP-8.

The first commit is simply Google's AIP-8 copied verbatim.

The second commit contains my changes to covert to AEP.

• Mechanical changes of aip to aep
• Straightforward changes or removals of "google"
• Some small additions such as requiring OpenAPI design examples and an explanation about case conventions.

[mkistler]

@rofrankel ready for re-review