feat: Add default languages list in `atlas` (in discussion)

Add default Open edX system-wide languages to the atlas pull workflow to avoid updating 50 repos whenever default languages change.

1- No default languages 2- Store config within openedx-translations:translations/default-filter.yml which Atlas queries dynamically on each fetch

My favorate because it's maintainer friendly in case someone wants to fork openedx-translations. 3- Hardcode default languages within Atlas, keep it overridable via the --filter=ar,de,fr_CA argument
My least favorate because we want different versions of Atlas work with different versions of openedx-translations and this change impede this use case. 4- Hardcode as the ATLAS_FILTER environment variable which should be set by deployment maintainers
Adds more effort to maintainers.

Currently default languages are specified in three ways:

i18n_tool tx pull pulls relatively completely translated languages by default from Transifex
Makefile of many repos has default set of languages e.g. https://github.com/openedx/frontend-app-learning/blob/ea2d7ed83975c6ec1ca5b66308338cd6317d9f89/Makefile#L2
config.yaml which affectsi18n_tool tx pull' in many repos: https://github.com/openedx/edx-platform/blob/master/conf/locale/config.yaml#L8-L29

None of the approaches above is useful with OEP-58 and atlas because of its dependence on the transifex-client

openedx / openedx-atlas