Crypto: Support Azure Key Vault

[reinkrul]

Uses:

• Crypto: let Exists() return an error if one occurs #3127
• Crypto: alter Storage interface to create keys inside key store #3120
• Crypto: work on crypto.Signer instead of concrete private keys for signing #3129

[reinkrul]

@woutslakhorst can you review this?

My main concern is that we need to provide Azure Key Vault with a key name, for which I chose to use the verification method's key ID. But, Azure Key Vault only allows [a-z0-9-] in key names, so we have to "dumb down" the key name, replacing all illegal characters with -.
The alternative is having a SQL table which maps verification method IDs (key IDs) to keys in Azure Key Vault, but then mapping keys in Azure Key Vault back to DIDs becomes hard(er) again. So I think this is the lesser evil.

But, if we want to support creating keys in key storage that require the key thumbprint (e.g. did:nuts or did:jwk), we need that coupling again, since we must determine the key name for Vault before creating the key.

I also checked if we can rename the key in Key Vault, but that's not possible.

[gerardsn]

LGTM
just a comment on the panic comment