feat: add support for Knative

Knative is designed for serverless workloads on top of Kubernetes. [Knative Serving](https://knative.dev/docs/serving/) runs scalable containers based on ingress gateway as its network layer. Currently, a few ingress gateway implementations such as [Istio](https://istio.io), [Kong](https://github.com/kong/kong) and [Gloo](https://github.com/solo-io/gloo) are supported. APISIX ingress controller, however, is not among them.

This issue aims at exploring Knative ingress management mechanism and adding support for Knative. In other words, make APISIX ingress controller an alternative Knative network layer to manage ingress traffic.

Any suggestions or comments are welcome! And thanks @Yiyiyimu and @moonming for being mentors!

Start by reading the linked Knative Serving documentation and comparing its ingress management mechanism with the existing Istio, Kong, and Gloo integrations. Then inspect the APISIX ingress controller entry points for ingress traffic management. Done means APISIX ingress controller can serve as a Knative network layer, with corresponding implementation coverage and tests.

Written by the indexing model from the issue text.