[PORT] [Expression]Enable list concatenation for function "concat"

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#4066

concat:

1. If all items are lists, concatenate them with list mode.
   concat([1, 2], [3, 4]) -> [1, 2, 3, 4]

2. If there exists any item that is not a list, use string concatenation mode:
   concat('a', 'b', 1, 2) -> 'ab12'
concat('a', [1, 2]) -> aSystem.Collections.Generic.List 1[System.Object] // unreadable, best to avoid

3. If null exists, skip it
   concat([1, 2], null) -> [1, 2]
concat('a', 1, null) -> 'a1'

issue link: #4014

Changed projects

• AdaptiveExpressions
• AdaptiveExpressions.Tests