Exclude UI and Swagger UI in dependent projects

If Conduit gets imported as a library we would like to exclude the UI and swagger UI by default and, if possible, make it configurable so they can get included on demand. For UI it might be harder to include it, as the UI needs to be compiled first before it can be compiled into the binary. Swagger UI on the other hand is committed and will be included even if Conduit is imported as a library, so we need to figure out how to exclude it by default (maybe with the use of build tags).

The motivation behind this is that the UI and swagger UI make up a good chunk of the Conduit binary size. Other projects that import Conduit as a library and don't need these parts should be able to exclude them from their binary.

We should not reinvent the wheel and check if any existing Go project found an elegant solution.