Add support for loading signing key from Azure Key Vault

[skoruba]

Add support for loading signing key from Azure Key Vault

Check this sample code:
damienbod/IdentityServer4AspNetCoreIdentityTemplate#30

[berendhaan]

@skoruba We have incorporated Azure Kv generated signing keys in our installation of your (magnificent) admin UI.
It it still rough and needs some more configuration options, but it could be a start for you. Shall I create a PR so you can have look?

[skoruba]

@berendhaan - I would be perfect, could you please send a PR from dev branch?
Thanks!

[berendhaan]

Done! #660
I've cleaned some stuff up before I made the PR. Haven't tested this implementation but it should work since nothing major was changes.