Implement SaveAndMergeAsync method in editable base types

[rockfordlhotka]

There should be a better abstraction for use of GraphMerger in the form of a SaveAndMergeAsync method on BusinessBase and BusinessListBase. This enables UI code like:

await Model.SaveAndMergeAsync();

The implementation should be something like this (in a list type):

        public async Task SaveAndMergeAsync()
        {
            var saved = await SaveAsync();
            var merger = new Csla.Core.GraphMerger();
            merger.MergeBusinessListGraph<Family, Child>(this, saved);
        }

[github-actions]

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