Add conformance test for PartiallyInvalid Route

[robscott]

What would you like to be added:
As suggested by @sunjayBhatia, we should figure out how to add conformance test coverage for #2429.

Before we do this we need to determine some kind of value that is considered invalid by implementations but passes CEL validation. Unclear what that will look like, and this will definitely require broader discussion.

Once that's in place (copied from #2429 (review)):

• Add a new core feature, SupportsPartiallyInvalidRoutes (name tbd) and test that makes sure the status is set to true when there is a partially invalid route, likely ignoring reason/message
• Add a couple new extended features, SupportsPartiallyInvalidRoutes[DropRule|Fallback]
  Implementations would opt-in to the test they support
• We can modify a route to becoming partially invalid and test the invalid rule is dropped or the old configuration is respected, depending on what test is run
• Status checked for the right values depending on what test is run

Why this is needed:
The spec change in #2429 introduces a new requirement for all implementations. Although it won't be trivial, it would be good to have conformance test coverage for this.

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