tools: rewrite spec build in go

[jshufro]

The four commits present are each fairly atomic.

1a0daa3 moves rpctestgen into tools/cmd/rpctestgen and updates CI accordingly
0c09476 runs gofmt to fix a lint error
ced5a4d introduces a new tool in tools/cmd/specgen that generates the refs-openrpc.json file
7578428 updates the build script + ci to use the new tool, and deletes the superceded spec build logic from build.js

I used jd to ensure there's no functional diff between openrpc.json as generated by main and the new form:

$ jd -set openrpc.json openrpc.json.bak
$ echo $?
0

Somewhat annoyingly, the new files are in a completely different order, since it isn't feasible to roundtrip raw json in go in a way that retains the order. If we need deterministic output, we can sort the output in build.js