feat(api): export GenRoutesString function

[jaronnie]

I generate an api file in my scaffolding, but it will cause the routes.go file to be constantly overwritten. There is already a method to generate types.go files, but there is no routes.go file, so I want to add a method to generate based on api.Spec

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.03%. Comparing base (8690859) to head (3ab293f).
Report is 146 commits behind head on master.

Additional details and impacted files

see 255 files with indirect coverage changes

[jaronnie]

@kesonan help review