refactor: Separate storage into its own package

[shrink]

Closes #150

I do not believe this is a breaking change because the Caddy module will continue to execute via the init entrypoint which now relies on the storage package.

I tested this branch in my non-Caddy CertMagic project using replace and it worked as a drop-in replacement:

replace github.com/grafana/certmagic-gcs => github.com/shrink/certmagic-gcs v0.0.0-20230730155505-0a3798824df6

After merge, the package can be added to the CertMagic wik: https://github.com/caddyserver/certmagic/wiki/Storage-Implementations

[CLAassistant]

All committers have signed the CLA.

[shrink]

@hairyhenderson re: the lint failure, I guess we have 3 options:

1. Rename the storage package to something else
2. Rename StorageConfig to Config
3. Add a linting exclusion for stuttering on StorageConfig

I guess, on reflection, since Storage is now its own package, StorageConfig is redundant. As StorageConfig isn't exposed for Caddy consumers, renaming StorageConfig to Config would be backwards compatible -- I think!

That said, I don't have a particular preference -- happy to go with whatever approach is standard for this org.

[hairyhenderson]

• Rename StorageConfig to Config

IMO that's the best approach - go ahead with that!

[shrink]

IMO that's the best approach - go ahead with that!

done! The golangci tool passed locally so it should be good to re-run the Workflows now 🤞