Implement Istio http policy binding controller

[yolocs]

Fixes #595

Changes

• Introduced Istio security API (along with generated stuff) as a dependency - Istio 1.5 release security v1beta1 API
• Implemented the controller that listens to http policy bindings annotated with binding class = istio
  • The controller interprets HTTPPolicy to Istio RequestAuthentication and AuthorizationPolicy
  • RequestAuthentication validates JWTs
  • AuthorizationPolicy checks request metadata

Meaningful changes are at: pkg/reconciler/security/istio

Release Note

Added initial Istio policy binding implementation.
It requires Istio release >= 1.5.
Documentation: /docs/security/README.md (TODO now)

Docs

[knative-prow-robot]

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[yolocs]

/assign @nachocano

[nachocano]

@yolocs made an initial pass, added a few nits.
The code is really neat!

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-google-knative-gcp-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/security/v1alpha1/common_lifecycle.go	80.0%	62.5%	-17.5
pkg/apis/security/v1alpha1/common_types.go	Do not exist	88.9%
pkg/reconciler/security/istio/httppolicybinding/controller.go	Do not exist	100.0%
pkg/reconciler/security/istio/httppolicybinding/httppolicybinding.go	Do not exist	88.7%
pkg/reconciler/security/istio/httppolicybinding/resources/authorization_policy.go	Do not exist	100.0%
pkg/reconciler/security/istio/httppolicybinding/resources/request_authentication.go	Do not exist	100.0%
pkg/reconciler/security/subject_resolver.go	Do not exist	89.7%

[nachocano]

/lgtm

Thanks for adding this @yolocs!!! Looking forward to seeing this in action!!!