rpc registration changes for desktop

@ben-polinsky @calebmshafer , what do you think about the following breaking change:

Right now we have viewerRpcs in packages/apps/desktop-viewer-test/src/common/ViewerConfig.ts, which is in the common dir so it can be shared by both frontend and backend. Thoughts on:

renaming the prop on the desktop viewer side from additionalRpcInterfaces to just rpcInterfaces?
would we remove all default rpc registration in desktop-viewer-react, and move that up to the app level?
- in our default template and test app, we would just import viewerRpc from common and pass it into the useDesktopInitializer hook and Viewer component

the proposed changes are in https://github.com/iTwin/viewer/pull/258/commits/f957b352dcbc0cebdfdf547a0543523bf6c1634e, which i can drop if we don't like

iTwin / viewer

rpc registration changes for desktop #258