Handle the "not now, but maybe later" use case (in SyncConfig?)

[grigoryk]

Follow-up to #3579

Currently if you don't specify a SyncConfig, sync will be disabled and a sync scope won't be obtained during authentication.

However, for applications that are planning to include syncing in the future, or for applications that may enable/disable it based on some remote flags, that behaviour means that applications are forced to manually specify sync scope via applicationScopes escape hatch. Otherwise, users authenticated while sync was disabled will not have a sync scope, and will be asked to re-authenticate once sync becomes enabled.

This is a good example of an information leak, and an "unknown unknown".

SyncConfig could provide a flag to indicate that sync scope should be obtained regardless, while still allowing disabling syncing?

┆Issue is synchronized with this Jira Task

[jonalmeida]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1794219

Change performed by the Move to Bugzilla add-on.