feat(chore): allow chained storages

[darkweak]

Closes #344

[wchargin]

This is a breaking change released in a patch version. For instance, it
breaks caddyserver/cache-handler:

./configuration.go:160:9: cannot use c.DefaultCache (variable of type *DefaultCache) as configurationtypes.DefaultCacheInterface value in return statement: *DefaultCache does not implement configurationtypes.DefaultCacheInterface (missing method GetStorers)
./httpcache.go:240:35: s.SouinBaseHandler.Storer undefined (type *middleware.SouinBaseHandler has no field or method Storer)
./httpcache.go:264:25: s.SouinBaseHandler.Storer undefined (type *middleware.SouinBaseHandler has no field or method Storer)
./httpcache.go:274:35: s.SouinBaseHandler.Storer undefined (type *middleware.SouinBaseHandler has no field or method Storer)

Is there at least a migration guide?

[darkweak]

Hello @wchargin That's an internal behavior that doesn't affect end users, previous configurations are totally compatible with this change.

[wchargin]

I don't follow?

My understanding is that DefaultCacheInterface is a public type
exported by the github.com/darkweak/souin/configurationtypes package.
For instance, it's listed on pkg.go.dev. And this pull request
adds a new method to that interface.

Doesn't that constitute a breaking API change for end users of the
github.com/darkweak/souin Go module who have defined custom types that
were intended to implement this interface but no longer do? If it were
backward-compatible, I'd expect to be able to update from previous
releases of souin to this one without error, but, as you can see, that
is not what happens.