dnsdist: Optionally add ECS for cache lookup when all backends are down

[rgacogne]

Short description

Add an option so that we add ECS information to the query before looking up the cache, even if no backend has been selected. This is useful when all backends in a pool use ECS, causing the cached entries to be hashed with ECS information.
Fixes #6098.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)