RTD modules: reprocess config on config change

[patmmccann]

On #15116 pr we observed:

I believe this needs to call `initSubmodules` as the bot suggests, but there's a complication: for some reason realTimeData accepts configuration only once

https://github.com/prebid/Prebid.js/blob/88fee5ceb7113b0db0828f12470fb37014f1dfd2/modules/rtdModule/index.ts#L98

which in turn means that rtd modules are normally initialized only once, and it's possible that some of them wouldn't work as expected if initialized again. The easiest way to do this IMO would be to keep track of which modules were already initialized in initSubmodules and call it each time a submodule is registered.

It seems we may also want to accept config multiple times