[PORT] Add new function in expression

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3470

close #3465
close #3462
add new function flatten and unique
add object support for indicesAndValues

Changed projects

• AdaptiveExpressions
• AdaptiveExpressions.Tests

[LG]

[Stevenic]

Needs porting

[Danieladu]

port in #1824 and #1813