Supporting iterate string by split function

[cosmicshuai]

closes #3279

Now split function can take a single parameter:
split('hello'), new string[] { "h", "e", "l", "l", "o" }

Also if the seperator is a null, it will return same result as the seperator is string.Empty. If both input string and seperator are string.Empty or null, it will return a empty string array.
split('hello',''), new string[] { "h", "e", "l", "l", "o" }
split('',''), new string[] { }
split('hello',nullObj), new string[] { "h", "e", "l", "l", "o" }

If the input string is null, it will return string.Empty regardless the content of the non-empty seperator.
split(nullObj,'e')", new string[] { string.Empty }

[coveralls]

Pull Request Test Coverage Report for Build 103241

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-2.01%) to 79.587%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.Dialogs/ObjectPath.cs	1	86.46%
/libraries/Microsoft.Bot.Builder.Dialogs/Prompts/OAuthPrompt.cs	1	56.4%
/libraries/Microsoft.Bot.Builder/Inspection/InspectionMiddleware.cs	1	78.89%
/libraries/Microsoft.Bot.Expressions/LRUCache.cs	1	82.93%
/libraries/Microsoft.Bot.Expressions/TriggerTrees/Trigger.cs	1	85.09%
/libraries/Microsoft.Bot.Builder.Azure/CosmosDbStorageOptions.cs	2	75.0%
/libraries/Microsoft.Bot.Builder/MemoryTranscriptStore.cs	2	86.09%
/libraries/Microsoft.Bot.Expressions/BuiltInFunctions.cs	12	93.21%

Totals
Change from base Build 103006:	-2.01%
Covered Lines:	10870
Relevant Lines:	13658

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💛 - Coveralls

[fuselabs]

✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.Luis.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.AI.QnA.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.ApplicationInsights.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Azure.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Dialogs.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.ApplicationInsights.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Integration.AspNet.Core.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.TemplateManager.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Builder.Testing.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Configuration.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Connector.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Schema.dll compared against version 4.6.3
✔️ No Binary Compatibility issues for Microsoft.Bot.Streaming.dll compared against version 4.6.3