Looks interesting PoC. I have written multiple extensions for PostgreSQL in Go. The hardest part is not "glueing" together the tools to make it work. Most of the time it's much more easier and faster to just write in C (C++ with wrapper).
There are many cases you should consider, like returning rows (composite types) and implementing trigger functions.
A different approach I wrote in rust
With some example consumers: https://github.com/durch/google-bigtable-postgres-fdw https://github.com/posix4e/jsoncdc