Support initialization hooks

[LikeTheSalad]

Some tools might require to do some work as part of the OTel RUM initialization process, this could be done in different ways such as by creating wrapper initialization APIs for example, although it seems to me that it could be easier/more convenient for the users of this SDK to always do the initialization process the same way regardless of any external tools they might need to use from any vendor that might need to add vendor-specific capabilities top.

Based on that, I believe we should provide some "hooks" for extensions to be able to get initialized as part of this SDK's RUM initialization process.