Poc of block broadcasting: Closes #244. It is only a PoC, as the workers simply broadcast blocks. There is no scheduling  (aura)  and received blocks by other workers are only logged to the command line without applying the state diff.

Major:
* Added `enclave-api` crate. PoC on how we can abstract the interface to the enclave.
* Added `Worker` struct. PoC on how we could abstract every aspect of the worker's features.
* Added PoC of a new `ws-server` the worker listens for direct requests. It comes from the [jsonrpsee](https://github.com/paritytech/jsonrpsee) crate - the rpc abstraction that will be successor to the current implementation in substrate.  It currently coexists with the old direct_server, but I think we should replace the old one soon. It removes a lot of boilerplate code and is async. Since the introduction of the `async/await` syntax in rust, I believe we should go async, as it is much less verbose in general.
* Added `Stf::apply_statediff()` it is unit tested but not yet integrated in the enclave.

Minor:
* better naming in the `direct_client` that actually reflects the functionality.
* Skip-ra does register the enclave with the registry but without remote attestation report instead of skipping the process completely. This was made possible in integritee-network/integritee-node#20
* Fixed minor stuff in the local-setup. Local setup stably works for me with a node that skips remote attestation checks.