Grpc ServiceProvider

[eric-feng-2011]

This PR updates the ServiceProvider with a new AES_GCM decryption function and adds a user_cert to the shared_key_msg. Handling of the user_cert and new encryption/decryption functions based on a client key are in src/enclave/Enclave/ but not included in this PR.
It also includes various wrapper functions that can be called by external python programs using ctypes. Finally, there is an updated CMakeLists.txt to create the necessary .so file for the aforementioned wrapper functions.

The ServiceProvider is important in steps 2 and 7 in the below.

In gRPC, the workflow is as follows:

1. RPCListener starts spark shell
2. Client performs RA with the enclaves
3. Client sends query to RPCListener
4. RPCListener sends query to enclaves which executes it. The encrypted results (encrypted under enclave key) are saved in a file(s) on the RPCListener
5. The client asks for post-verification
6. The RPCListener performs post-verification and then re-encrypts the results under the SP key and then returns it
7. The SP decrypts the encrypted values returned using its own key

The pull requests should be done in the following order:
Key-get -> gRPC reEncryption -> gRPC ServiceProvider -> gRPC Python Files