[keyserver] add flow types for grpc client

Summary: adding the relevant types for the grpc client. I had to use any/mixed a few times to avoid pulling in more dependencies like the events library and http2.

Test Plan:
ran flow in keyserver
also, after the subsequent diff, I added the following code to keyserver.js and re-ran flow:

+  const user = { userID: '12345', deviceID: 'ahhhhh' };
+  const publicKey = identityClient.getUserPublicKey(user, (e, o) => {
+    if (e) {
+      console.log(e);
+    } else {
+      console.log(o);
+    }
+  });
+  console.log(publicKey);

also ran yarn dev and got back the expected public key

Reviewers: atul, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5609