We want to export some of our CLI code to re-use in other projects. But
in Golang you cannot import code from a `main` package.
So we need to move the actual code into its own package and only have
the `func main()` in the `main` package.
This commit swaps out golang/protobuf/jsonpb for a custom variant that
by default prints byte slices as hex, which is more useful for our
setting. Some existing wrapper structs are removed as they can now be
printed directly with the new jsonpb.
!!! NOTE !!!
This commit introduces a breaking change to lncli listinvoices since
payment hashes and preimages will now be printed in hex instead of
base64.
