New feature: enable Vault proxy capabilities in injected Vault Agent sidecar.

Applications will be able to invoke any Vault API endpoint through this local proxy without having to deal with authentication and token.

Tasks:

• Update documentation
• Add new blog entry
• Add new samples
• Add new annotations to enable proxy mode and, as an option, provide custom proxy port

Proxy capability will be added as a new Vault Sidecar Injector mode. We will end up supporting two modes:

• proxy, providing local proxy to Vault server
• secrets, enabling dynamic and continuous retrieval of secrets

It will be possible to enable only one or both modes on a case by case basis for each mutated pod.