Improve CLI use case

[twifkak]

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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 + openssl can be used for this.)
• [ ] Maybe ACME cert renewal (unless we can confirm an existing CLI works for SXG certs).
• [ ] Anything else?

[twifkak]

cc @ithinkihaveacat @uskay