Make runtime host API more flexible

[kostko]

In order to make the runtime host API more flexible to support different execution models in the future, we should refactor the existing worker host into the following parts:

• go/runtime/host/protocol (from existing go/worker/common/host/protocol)
• go/runtime/host/manager.go (common part of go/worker/common/host/sandboxed.go that manages Oasis Core Runtime Enclaves, regardless of what sandboxing/confidentiality platform they use)
• go/runtime/host/sgx (provisioner for Intel SGX using Bubblewrap and runtime-loader).

While refactoring the API we should consider use cases where the runtime abstraction (VM/enclave) is pre-provisioned (e.g., externally provisioned Oasis Core VM and one or more Oasis Core Runtime VMs together with some interconnect).

In addition to the runtime host protocol channel there should also be a separate channel (pipe) for logs from the runtime (related to #1819). The actual transport should be abstracted.