Enable debug build

[duncanpo]

The current cmake build only supports release build. To build debug, a few things have to be done manually:

1. Debug runtime libraries need to be pulled in instead of release versions (e.g. libcurl-d.dll)
2. Debug flag needs to be propagated to underlying opentelemetry-cpp library
3. Symbol files (.pdb on Windows) need to be installed/copied to the right place

All these should be done automatically by the cmake script and no manual steps should be necessary.