[web] add flow types for grpc-web unary interceptor

Summary:
added the relevant types using this file as reference: https://github.com/grpc/grpc-web/blob/master/packages/grpc-web/index.d.ts

Depends on D9670

Test Plan: ran flow

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, wyilio

Differential Revision: https://phab.comm.dev/D9671