Add ability to modify top level log strucutre

[pshrote-afk]

line 104 src/Sinks/AzureLogAnalytics/AzureLogAnalyticsSink.cs

Issue: Custom log structure is only available against "Event" field.

Suggestion: This package will be more flexible if log structure can be modified at the top level.

Example:

1. Current limitation for log structure:
   [{
   "TimeGenerated": xyz,
   "Event": [{Userid: x}, {EntityId: y}, {Action: z}],
   "Message": "xyz"
   }]

var logs = logEventsBatch.Select(s =>
                {
                    var obj = new ExpandoObject() as IDictionary<string, object>;
                    obj.Add("TimeGenerated", DateTime.UtcNow);
                    obj.Add("Event", s);
                    obj.Add("Message", s.RenderMessage());
                    return obj;
                });

2. Proposed flexibility:

[{
"TimeGenerated": xyz,
"Event": [ ],
"UserId": x,
"EntityId": y,
"Action": z,
"Message": "xyz"
}]