The Auto-generated proto.ts should have async methods that returns promises

[lhr0909]

loopbackio/loopback4-extension-grpc#31

We were evaluating if we could expose loopback v4 endpoints as gRPC endpoints, and found that the .proto.ts part of the code during auto-generation is not building methods that returns a promise. Currently we can have synchronous returns, but in the NodeJS world, we need these controller methods to be async so we can fetch data and return.

[lhr0909]

#2