Add support to get only metadata from the triggers

_From @pragnagopa on April 17, 2018 17:49_

Please provide a succinct description of the issue.

Scenario requested in [Azure/azure-functions-host/issues/2301](https://github.com/Azure/azure-functions-host/issues/2301)

When binding to a CloudBlockBlob in a C# function, user can control when to read stream attached to the binding and if needed can be skip it completely.

It is worth adding this support for other languages in V2 as language workers as of now only have support to bind to basic data types: bytes, string and json

_Copied from original issue: Azure/azure-webjobs-sdk#1667_

Start by tracing the V2 language-worker binding path for CloudBlockBlob and the existing bytes, string, and JSON binding types. The issue provides no file or test entry point; done means other languages can obtain trigger metadata without reading the attached stream.

Written by the indexing model from the issue text.