Add release automation to keep in sync with Istio release

[bianpengyuan]

Wasm extensions in this repo will be released out of band, but keep in sync with Istio release so that they can be easily used in Istio docs without version incompatibility concern. This issue is to track the work to automate release process which

• Watches Istio new branch/tag, and create the same branch/tag correspondingly.
• Push Wasm modules with the same Istio tag.

The same release process should be reusable for users who fork or clone this repo for their own extensions.