Improve CLI use case

Some folks are using `cmd/transform` + `gen-signedexchange` offline, rather than running `cmd/amppkg` online. Improve this use case, filling in the gaps of the other things amppkg does with offline alternatives:

- [ ] Adding a `-version` flag to `cmd/transform` so it can output older versions.
- [ ] Specifying the proper inner headers (perhaps output as flags to `gen-signedexchange`).
- [ ] Specifying the proper outer headers. (Is there a way to do this e.g. with static files on Apache or nginx?)
- [ ] Perhaps knowing when the OCSP response needs to be refreshed. (Though [rq](https://github.com/rq/rq) + `openssl` can be used for this.)
- [ ] Maybe ACME cert renewal (unless we can confirm an existing CLI works for SXG certs).
- [ ] Anything else?

Start by reading the cmd/transform, gen-signedexchange, and cmd/amppkg entry points and comparing the offline workflow with the online one. Review the listed gaps around version selection, inner and outer headers, OCSP refresh, and ACME renewal; done requires a decided scope and working offline alternatives for the selected capabilities.

Written by the indexing model from the issue text.