port: [Debugger] add injectable transport, with unit tests (#4921)

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The changes in [Debugger] add injectable transport, with unit tests (#4921) may need to be ported to maintain parity with microsoft/botbuilder-dotnet.

## Description This PR allows the debugger transport to be injectable to 1. simplify the debug adapter code 2. remove implementation inheritance 3. allow unit tests (with mocked transports) for the debug adapter code

Specific Changes

• create new IDebugTransport interface
• move code to use IDebugTransport interface
• add first substantial unit test for debug adapter

Testing

Yep.

Please review and, if necessary, port the changes.