gRPC reencryption code

[eric-feng-2011]

Code for allowing re-encryption of query results. This corresponds to steps 4, 5, and 6 of the gRPC workflow as mentioned 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

This PR would NOT work upon pulling as it depends on another PR being completed first (key-gen).

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