Expose AbsoluteConfigKey prepending/appending

[finnvoor]

Motivation

Docs mention:

// Only remap the environment variables, not the JSON config
let keyMappedEnvProvider = KeyMappingProvider(upstream: envProvider) { key in
    key.prepending(["myapp", "prod"])
}

but prepending(_:)/appending(_:) were internal.

Modifications

Switched internal to public.

Result

KeyMappingProvider is easier to use :-)

Test Plan

n/a

[czechboy0]

Thanks for the PR, @finnvoor - I'll take a look next week, attending ServerSide.Swift in London this week.

[czechboy0]

Thanks @finnvoor - I agree it'll be useful to make these public, let's just duplicate the first one and only publicize a non-optional variant.

[czechboy0]

Could you add a few unit tests for these two methods please? To make sure the path and context get combined correctly.