Workspace Key Vault access for newly added silos

[thomasp-ms]

When adding a silo to an already existing setup, the new computes will not have access to the default workspace key vault. We need to update the doc and explain how to do this manually (give the "Key Vault Crypto User" role to the compute's managed identity), or modify the bicep templates accordingly (we would need to introduce an extra argument, the key vault name).