Support Rate Limiting feature in OSSM 2.1.x

OSSM 2.0.x was built on upstream istio 1.6 and we tested Rating Limiting case by
1. Enable SMCP Policy Mixer Plugins:
$ oc patch -n istio-system smcp/basic --type merge -p '{"spec":{"policy":{"type": "Mixer", "mixer":{"enableChecks":true}}}}'
2. After SMCP reconciliation completes, follow upstream istio 1.6 Rate Limiting doc steps (memquota example):
https://istio.io/v1.6/docs/tasks/policy-enforcement/rate-limiting/

The incoming OSSM 2.1.0 will be built on upstream 1.9 or above. The way of configuration will be changed. It's not clear how we can support Rate Limiting in OSSM 2.1.0.

• Need to investigate an approach we will support.
  • Should we use EnvoyFilter as a workaround for 2.1.0, like upstream 1.9, or should we define a proper solution?
• Need to add our implementation of configuration in OSSM 2.1.0 if needed
• Need to change our way of testing Rate Limiting in 2.1.0 test cases.