cli: Generate Go types and bindings in an adjacent file

[sdboyer]

Currently, the Go types and bindings generator in the CLI generate to stdout. This isn't terribly idiomatic - proto sets the general pattern here, with generating an adjacent file. It's probably generally more convenient for callers, anyway.

This is especially problematic for bindings, though, because the generating an embed.FS that will reliably work when passed to load.InstancesWithThema (or any such function we could write) requires being able to inspect the disk environment where the generated file will land. That's impossible on stdout.

So we'll have both of those generators default to generating an adjacent file (<name>_types_gen.go, <name>_bindings_gen.go), and if bindings are forced to stdout, just not allow an embed to be generated at all - it's up to the caller to make it sane.

[sdboyer]

cc @marefr - taking your advice on this one :)