Make an authorization middleware for Envoy based on Casbin

[hsluoyz]

See:

1. https://www.openpolicyagent.org/docs/latest/envoy-introduction/
2. https://github.com/open-policy-agent/opa-envoy-plugin
3. https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/security/ext_authz_filter.html
4. https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/security/rbac_filter
5. https://github.com/envoyproxy/envoy-filter-example

This issue originates from: #50

[hsluoyz]

@EmperorYP7