[new feature] Connection with Identity - Issue #73

[kumkee]

This is to implement the new feature suggested in Issue https://github.com/aaronpowell/FSharp.CosmosDb/issues/73 - connection via default identity instead of keys.

~Disclaimer~

~The addition has not been tested as I am still learning how to locally build and test a Nuget package. Any help will be appreciated. So far, I've only read about Nuget local feeds but have no clues on how to compile the whole repo into a .nupkg file.~

It is supposed to work as follows

let findUsers() =
    host
    |> Cosmos.host
    |> Cosmos.connectWithIdentity
    |> Cosmos.database "UserDb"
    |> Cosmos.container "UserContainer"
    |> Cosmos.query "SELECT u.FirstName, u.LastName FROM u WHERE u.LastName = @name"
    |> Cosmos.parameters [ "@name", box "Powell" ]
    |> Cosmos.execAsync<User>