OpenAPI generator use case: recurring vs. one-time

## Suggestion

The current [OpenAPI generator](https://loopback.io/doc/en/lb4/OpenAPI-generator.html) seems like it's meant to be used once, upfront, to scaffold code.

My suggestion is to (also/instead) support the use case where you run the generator regularly as part of your build, and don't even check in the generated files. (this is how I generally prefer to use code generators)

Was this considered in the design of the current generator? Is it possible to achieve something like this with LoopBack today?

## Details

I'm starting a new project, and I expect to gradually evolve the schema over a couple years. Here's how I want it to work when a developer is updating application code to account for changes in the schema:
- pull down the new schema
- run the application build (this runs the code generator, replacing all the generated files)
- application code that references the generated files now has TypeScript errors, showing you where your attention is needed. (follow the squiggles)

Other desirable properties of code generators:
- Generated files have a recognizable, git -ignorable ending (e.g. `.gen.ts`)
- There's a big scary header comment at the top saying "THIS IS A GENERATED FILE, DO NOT EDIT"
- Ideally the generated code is formatted, at generation time, according to the project's prettier config. (for example, see [graphql-code-generator](https://graphql-code-generator.com/docs/integrations/prettier))
- You never need to make a change in a generated file. Instead you make changes in the generator's _input_. (e.g. the OpenAPI schema or in the generator configuration)
- Because the generated file is unversioned and replaced wholesale, you're never asked to resolve a merge conflict between the new template and your project-specific changes.

## Alternatives

- Is Loopback's OpenAPI generator built on top of other libraries or tools that I might be able to use separately?
- Do you know of other node.js web frameworks that might align with this philosophy? I don't need all the features loopback provides, I'm just hoping to make my OpenAPI schema the source of truth for endpoint declarations, input/output types, etc.

Start with the linked OpenAPI generator documentation and inspect how LoopBack currently runs the generator. Compare that behavior with the requested recurring-build workflow, including unversioned generated files, generated-file markers, formatting, and TypeScript feedback. Done would require an agreed design and a supported way to use the generator from application builds.

Written by the indexing model from the issue text.