Add pRPC AllowHandoverTo

[kvinwang]

This PR adds a pruntime RPC named AllowHandoverTo. It allows the council members to sign a new version of pRuntime in order to enable them to hand over the key when pRuntime encounters issues syncing block data.

This is part of #1499.

Usage

Suppose we have two version of pRuntime A and B, where A is stucked, and we want to force handover to B.

SGX MR of A: 0x10c24c0e6bf8a86634417fcd8f934e62439c62907a6f1bc726906a50b054ddf10000000083d719e77deaca1470f6baf62a4d774303c899db69020f9c70ee1dfc08c7ce9e
SGX MR of B: 0xf42f7e095735702d1d3c6ac5fa3b4581d3c3673d3c5ce261a43fe782ccb3e1dc0000000083d719e77deaca1470f6baf62a4d774303c899db69020f9c70ee1dfc08c7ce9e
Genisis block hash: 0x0a15d23307d533d581291ff6dedca9ca10927c7dff6f4df9e8c3bf00bc5a6ded (Can be got via get_info)

Then the steps would be:

1. Ask at least half of the council members to sign a message as below:

Allow pRuntime to handover
 from: 0x10c24c0e6bf8a86634417fcd8f934e62439c62907a6f1bc726906a50b054ddf10000000083d719e77deaca1470f6baf62a4d774303c899db69020f9c70ee1dfc08c7ce9e
 to: 0xf42f7e095735702d1d3c6ac5fa3b4581d3c3673d3c5ce261a43fe782ccb3e1dc0000000083d719e77deaca1470f6baf62a4d774303c899db69020f9c70ee1dfc08c7ce9e
 genesis: 0x0a15d23307d533d581291ff6dedca9ca10927c7dff6f4df9e8c3bf00bc5a6ded

See https://files.kvin.wang:8443/signit/ for an example

2. Collect the signatures and assamble them into a rpc request like this:

   $ cat sigs.json
   {
       "measurement": "f42f7e095735702d1d3c6ac5fa3b4581d3c3673d3c5ce261a43fe782ccb3e1dc0000000083d719e77deaca1470f6baf62a4d774303c899db69020f9c70ee1dfc08c7ce9e",
       "signatures": [
           {
               "signature": "fe6eeb25c088975df9bd136cc29c01a1b0bec3c4a58027efd7ca2908b233983c908a7159b81e265948a45e2c9129560f96aef24b93612f1dd4fc9aa40880ff88",
               "signature_type": 4,
               "pubkey": "d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d"
           },
           {
               "signature": "22591a9f308e9d1a2af2ad103334cf8ab3674a2dab9e9a6372cf1e09c8671066668ed90af1c88ad7c5c280b8e5dfb043402774cf59e38d312ee107bd8aee2f8c",
               "signature_type": 4,
               "pubkey": "8eaf04151687736326c9fea17e25fc5287613693c912909cb226aa4794f26a48"
           }
       ]
   }
   

3. Load the sigs.json to pruntime A

   $ curl -d @sigs.json  localhost:8000/prpc/PhactoryAPI.AllowHandoverTo?json
   

4. Run a new pruntime B instance to start the handover
   $ ./gramine-sgx pruntime --request-handover-from http://localhost:8000

[shelvenzhou]

LGTM