Add support for Azure Blob Storage

[joe-elliott]

Is your feature request related to a problem? Please describe.
Tempo currently supports s3 and gcs. It would be nice if it supported Azure as well.

Describe the solution you'd like

Describe alternatives you've considered

Additional context
These are the interfaces that must be implemented to add a new backend: https://github.com/grafana/tempo/blob/master/tempodb/backend/backend.go

[calvernaz]

The Azurite emulator can be useful to test against this backend.

https://docs.microsoft.com/en-us/azure/storage/common/storage-use-azurite

[pedrosaraiva]

We are currently using Grafana and Loki in our platform, so Grafana tempo seems the right fit for us in the near future.
I made a WIP PR to implement Azure storage since run on Azure would be a requirement for us. :)