sg: add generate command

Created by: jhchabran

Merge https://github.com/sourcegraph/sourcegraph/pull/32381 first. Fixes https://github.com/sourcegraph/sourcegraph/issues/25441

TODO

• Rename sg generate into sg generate go or something else?
• Update CI scripts Covered by https://github.com/sourcegraph/sourcegraph/issues/29052

Notes: I tried running the generate commands concurrently, it's only a very marginal improvement, probably not worth it (less than 6secs on my machine)

Test plan

• run tests locally
• CI