Extension configuration handling: optimization opportunity

When the code for handling effective configuration in extensions is [consolidated]( https://github.com/envoyproxy/nighthawk/issues/498) there might be an opportunity
to optimize:

Each of our extensions independently read, validate and compute effective configuration based
on request level configuration. This is good, because each on should be able to function
independently. However, in the likely case that all of them are enabled, there should be a way to
cache the results of the first extension and carry that across to extensions running after it for
re-use, to eliminate redundant work.