This PR implements the planned merge of envoyproxy/data-plane-api into envoyproxy/envoy as described in #2934 and https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!topic/envoy-dev/KcVHFH-zQwQ

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Risk Level: Medium (there might be unintentional breakage of dependent builds).
Testing: CI passes. There is now an additional bazel.api do_ci.sh target to build and run API tests.

Fixes #2934.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 1fdb6386c4bb42748530d7a9bf58ded644d77749