Conformance test for multiple mirror filters within the same rule

[LiorLieberman]

What would you like to be added:
Conformance test for multiple mirror backends

Why this is needed:
#2199 enables support for multiple HTTPRouteFilterRequestMirror filters in the same HTTP Rule.

We should add SupportFeature and conformance test for it.

Support level would be implementation-specific.

[robscott]

Support level would be implementation-specific.

I think support level here would actually be extended since we'd expect behavior to be consistent across the implementations that support mirroring to more than one backend.

[shaneutt]

This is a feature we already support, so we definitely need this.

/triage accepted

We should get this done for GA.

/milestone v1.0.0
/label release-blocker

[levikobi]

I would like to take this if nobody is working on it

[shaneutt]

Nobody is, thank you we really appreciate the help getting GA done 🙇

/assign @levikobi

Let us know if you get stuck or need help, we're hoping to have this and all other release blockers complete within the next 4-6 weeks.