[web] Introduce types for identity service communication

Summary:

1. I added the latest version of google-protobuf to web/package.json
2. I installed its Flow types via flow-typed install google-protobuf
3. I generated web/protobufs/identity-client.cjs and web/protobufs/identity-structs.cjs by running protoc -I=shared/protos/ identity_client.proto --js_out=import_style=commonjs:web/protobufs and then added the @generated line to hide them from Phabricator diffs
4. I wrote web/protobufs/identity-client.cjs.flow and web/protobufs/identity-structs.cjs.flow, which were inspired by TypeScript bindings generated with protoc -I=shared/protos/ identity_client.proto --grpc-web_out=import_style=commonjs+dts,mode=grpcwebtext:.
5. I wrote flow-typed/npm/grpc-web_v1.x.x.js, only typing the things that were needed for the last step

Test Plan: I tested the code and the Flow types with this patch: https://gist.github.com/Ashoat/7cf4a0fbdebc152d585884184bc0ac07

Reviewers: varun, jon, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7306