[Refactor] Common place for types/interfaces/constants/helpers shared between client and server side

[yizheliu-amazon]

Move types/interfaces/constants/helpers shared between client and server side as many as possible into single place, to avoid duplicate codes for better maintenance.

[ohltyler]

As @yizheliu-amazon has mentioned in #329, we will want to move any shared constants on the server side, to prevent the issue of missing constants when the user builds and runs the artifact, since the client-side source files are ignored when building (details here)