adding FetchInvoice grpc call to node proto

The Rust [grpc node api](https://github.com/ElementsProject/lightning/blob/master/cln-grpc/proto/node.proto) does not expose a call to `FetchInvoice`, despite being present in the [schemas](https://github.com/ElementsProject/lightning/blob/master/doc/schemas/fetchinvoice.schema.json) and not omitted in the patch. 