feat(nimbus): fetch targeting contexts directly from each firefox app source code

[moibra05]

Because

• We plan to add automated validation for Experimenter targeting configs by checking JEXL field references against the upstream targeting context definitions in the Desktop, Fenix, and iOS source code
• Manifesttool already fetches app manifests per ref/version, but it was not downloading the targeting context source files that external config updates depend on.

This commit

• Adds targeting_files to app config and configures it for firefox-desktop, fenix, and ios
• Stores the files by version in each app's feature manifest as well as an unversioned copy in the root

Fixes #14922