Support managed identity for authenticating a function to the storage

#### What problem would the feature you're requesting solve? Please describe.
Today, in order to configure a blob input/trigger, you need to provide a connection string. Managing credentials is complex, so I would like to leverage the function managed identity instead.

#### Describe the solution you'd like
Use function managed identity to authenticate the app to the storage account. This makes the credentials transparent to the user and is more secure compared to the current solutions.

#### Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

#### Additional context
Add any other context or screenshots about the feature request here.

Start by tracing how Azure Functions blob inputs and triggers currently use connection strings for storage authentication. The issue names no files or tests, so first identify the relevant host entry points and existing managed-identity support; done means a defined, tested way for functions to authenticate to the storage account without a connection string.

Written by the indexing model from the issue text.