Add Inigo test to ensure we are not using any deprecated Envoy API fields

[sunjayBhatia]

As a follow up to #526

We thought it would be useful to add an Inigo test that curls the Envoy admin server and ensures the config dump does not include any deprecated fields (see #521 for an example of what it would show).

We didn't do it as part of #526 as it is a little involved and we wanted to keep that PR small, the flow should possible go something like this:

• modify Inigo test app (grace? or another?) to have the ability to send a request to the Envoy admin server
  • could be a new endpoint, it may return the content back as a response to the request it received
• wait for LRP to be running
• make a request to the app to fetch the Envoy config dump
• check config dump has no fields that are deprecated
  • substring checking? only check the keys? or could parse the config into the actual structure or some generic map

This test would start to fail when we bump Envoy and are still using fields that are newly deprecated so we can fix them ASAP, block us releasing, etc. rather than find out after the fact from issues like #521

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[mdelillo]

I pushed a WIP branch here. It has the latest release of go-control-plane and udpa, but the test is failing because hidden_envoy_deprecated_build_version is present in the config.

[sunjayBhatia]

Hmm, the deprecated field thing may be because of the following:

bootstrap config (the base of the envoy config) has a node element: https://github.com/envoyproxy/envoy/blob/79d7d4ee917c4faff78b5ae8aa1b07cf2ff91cfc/api/envoy/config/bootstrap/v3/bootstrap.proto#L109

the Node type has a version: https://github.com/envoyproxy/envoy/blob/79d7d4ee917c4faff78b5ae8aa1b07cf2ff91cfc/api/envoy/config/core/v3/base.proto#L182

However it seems b/c we're on v3 and the v2 version is referenced as the previous version/theres a reserved field, the proto dump generated may be adding this deprecated field: https://github.com/envoyproxy/envoy/blob/79d7d4ee917c4faff78b5ae8aa1b07cf2ff91cfc/api/envoy/config/core/v3/base.proto#L137-L141 and https://github.com/envoyproxy/envoy/blob/79d7d4ee917c4faff78b5ae8aa1b07cf2ff91cfc/api/envoy/api/v2/core/base.proto#L166

I can't see a good way to get rid of that field from the config dump, except maybe specifying our own envoy user agent, which is not really relevant to how we use envoy