feat: add --namespaces support to sync

[gu-stav]

Add --namespaces configuration for sync, which has been discussed in #137.

With this new option is becomes possible to define for the sync command, which namespaces are handled by it.

If namespaces are defined:

• New keys, which do not have a namespace attached, are not pushed anymore.
• New keys, which do not belong a to defined namespace are not pushed.
• Existing unused keys, which do not belong to a defined namespace are no longer deleted, when sync is used with --remove-unused.

The latter makes it possible for multiple applications to use the same project without knowing about all namespaces within that project, as outlined in the issue: #137

Builds on top of #139

[stepan662]

I've tried to finish this here #151 , but I see an inconsistency with the push command. Where push command's --remove-unused removes all other keys (regardless of the namespace). Here however we would only delete unused keys within the given namespace.

This kind of inconsistency is potentially very confusing for the user. I think we could name it e.g. --namespaces-scope, or something like that. And also add --namespaces, but keep the same logic as is in push command. Later there could be also --namespaces-scope for the push command (the logic is implemented on BE, so it would be a bit more complicated).