Add support for Managed Identity access

[jayendranarumugam]

Provided the support for Managed Identity access for the SQL.

[Jandev]

Thanks, this is looking like a nice first step.
The biggest issue I have with this is the dependency on the Microsoft.Azure.Services.AppAuthentication package, which makes this binding a bit bigger for only the Managed Identity scenario.

In order to solve this I was thinking of somehow 'injecting' the functionality to retrieve a token, so the client can take the dependency on a specific version of the Microsoft.Azure.Services.AppAuthentication package (or something else for that matter), as long as a valid Access Token is generated.

Something like:

interface ISqlBindingTokenProvider
{
    Task<string> GetAccessToken();
}

If the client (Function App) has a type registered with the ISqlBindingTokenProvider interface, the following can be implemented

using (var connection = new SqlConnection(attribute.ConnectionString))
{
	// Somehow retrieve the ISqlBindingTokenProvider and if not `null`, invoke the method `GetAccessToken`
	// and add the retrieved value to the `connection
	connection.Open();
	using (var command = connection.CreateCommand())

I haven't done the research on how to inject such a thing though.

I'll add this also to the open issue to make the expectation more clear.
All of this 'complexity' is to make sure the package is as small as possible and we don't introduce dependencies to package versions which the client might not want.

[jayendranarumugam]

Open a fresh PR #12 . So closing this old one