NewInvoker inits the invoker.

NewProtoInvoker inits the invoker.

Invoker invokes a lambda using "I" as the input type and "O" as the output type.

ProtoInvoker invokes lambdas that use protobuf encoding.

Lambda is implemented by the official SDK's client.

HandlerFunc implements lambda.Handler, similar to http.HandlerFunc.

JSONHandlerFunc implements lambda.Handler but decodes and encodes as JSON.