Reduce time & effort required for rebasing to new istio & envoy versions

Goals

• reduce number of person-days required to rebase to new istio and envoy versions

To achieve our goal, we'll need to reduce the amount of effort required for a rebase by redesigning, restructuring or refactoring our code base and processes.

Rebase tasks include:

• create new branches
• write prow configuration/jobs
• add vendor/ directory in separate commit
• generate list of commits
• mark commits that out-of-date
• cherry-pick commits one by one
• submit PR that has the cherry-picked commits to run CI against the changes

Note: this epic only covers refactorings and redesigns that will help us to simplify our rebases. It does not cover rebase automation, which is tracked in OSSM-719.