[functionapp] Allow KuduLite decrypts assignment context

[Hazhzeng]

Background

The request is used by Linux Consumption functionapp server-side-build project. The project requires KuduLite to run on Service Fabric Mesh.

When deploying KuduLite in a Service Fabric Mesh container, KuduLite will receive an encrypted AppSettings context, we want KuduLite to decrypt the context and finish the initialization process.

Pull Request Overview

1. Introduce WEBSITE_AUTH_ENCRYPTION_KEY. This is used for generating an authentication token. The token is required when sending request to Antares Frontend.
2. Introduce Decrypt() function. This function can decrypt the HostAssignment context during the container assignment process.
3. Introduce HostAssignment context. It contains the basic information of the SCM site (e.g. site name, environment variables).

Code Review Request: @sanchitmehta @JennyLawrance @ankitkumarr @maiqbal11