API stability proposal

This is an extension of the conversation started in:
https://docs.google.com/document/d/1eDQQSxqx2khTXfa2vVm4vqkyRwXYkPzZCcbjxJ2_AvA/edit?ts=5af97a4e

Ultimately, we would like to stabilize the API and have a deprecation period like we do for config changes. Getting there is going to take a bunch of thought and a solid plan. We also need to make sure we don't reduce velocity when the project is still relatively young.

Related to https://github.com/envoyproxy/envoy/issues/827

cc @brian-pane @htuch @alyssawilk @envoyproxy/maintainers