[Port] make custom function easier

[cosmicshuai]

Fixes #1931

Make customized function in LG and expression easier by accepting just a lambda.

At the cost of expose FunctionTable instead of a generic IDictionary.

Now you can do

Expression.functions.add('contoso.sqrt', (args) => {
            let result = null;
            if (args[0] !== undefined ) {
                const inputFloat = parseFloat(args[0]);
                result = Math.sqrt(inputFloat);
            }

            return result;
        });

to add a new function

[coveralls]

Pull Request Test Coverage Report for Build 117193

• 19 of 36 (52.78%) changed or added relevant lines in 3 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.03%) to 81.55%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/adaptive-expressions/src/functionTable.ts	16	33	48.48%

Files with Coverage Reduction	New Missed Lines	%
libraries/botframework-streaming/src/webSocket/webSocketTransport.ts	2	80.65%
libraries/botframework-streaming/src/webSocket/webSocketServer.ts	7	66.67%

Totals
Change from base Build 117144:	-0.03%
Covered Lines:	9344
Relevant Lines:	11040

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💛 - Coveralls