[React RUM] media observers

[igor-grubic]

The current implementation only captures media mutations in a data-block.
This implementation adds a generic media observer for React containers.

Notes:

• This was a bit rushed and lacks more thought, however it could be useful for discussion purposes.
• When trying to solve the question of "when is react finished loading" i have not found a generic solution that does not require some kind of implementation on the react side, hence the observers on the react container
• I think that the proposed solution, depending on the size of the react app, can have performance impacts on the host app and should be tested a bit more in depth.
• another idea could involve using setTimeouts (or something in a polling method) instead, which would be less precise but less impactful performance wise
• Is the current implementation of data-block only going to change? If so, we might want to have a generic solution that solves both problems.