Add support for flexible subscriptions

[nielsenko]

Example:

    ...
    final realm = Realm(Configuration.sync(user, [Task.schema]));
    realm.subscriptions.update((mutableSubscriptions) {
      mutableSubscriptions.add(realm.all<Task>(), name: 'myTask', update: true);
    });

[coveralls]

Pull Request Test Coverage Report for Build 2310272717

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.677%

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Totals
Change from base Build 2307752907:	0.0%
Covered Lines:	400
Relevant Lines:	427

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💛 - Coveralls

[blagoev]

I will need more details on these changes. Why do we need to reference EventLoopDispatcher type from core. Lets have a discussion about this.

[blagoev]

why do we call it flexibleSync. Since we are supporting only that type of sync isn't it better to name it Configuration.sync

[nirinchev]

All our documentation and marketing talk about flexible sync. I think it'll be confusing if the dart SDK is the only one which doesn't specify the sync type. When/if we deprecate PBS, we can deprecate the "flexible" terminology across all SDKs.

[nielsenko]

I tend to agree with @nirinchev. I'll keep as is for this PR, we can always change later.

[blagoev]

I don't agree that docs and marketing should play here how we name this exact method. Yes we have "flexible sync" type but why put it in the config method name. We don't have any other sync. The docs could go into great detail how sync in Dart works but this should not be imposed on the method name.

[blagoev]

and we are keeping the FlexibleSyncConfiguraiton class name

[nielsenko]

Who can call this decision?