Support Azure Storage emulator client with sas tokens

[perfectayush]

Currently storage clients for emulator (azurite) can't be initilized with SAS tokens. KeyClient struct for emulator are created with hard-coded account key when with_emulator function is used. The blob endpoint can't be modified as these structs are private.

For writing integration tests against emulator which emulates production use cases of using SAS, it's better to allow more flexible client configuration supporting SAS token or allowing to modify blob endpoint.

I think, for better flexibility extending from_connection_string function makes more sense. Please suggest your opinion on this, I can contribute code for this