[rum] Inject browser sdk

[yoannmoinet]

What and why?

Automatically inject RUM's Browser SDK into the bundle.

How?

A few updates were necessary:

• Add auth to the doRequest helper, so we can fetch the correct clientToken in case the customer didn't add it to the rum's configuration.
• Improve logger and build report with more data on the bundler.
• Create a better isolation of the injected code using an IIFE.
• Update the bundling mechanism of our plugins to allow side builds declared by the plugins (ex: RUM needs to pre-bundle the browser SDK file into a single file).
• Implement the RUM plugin and add tests.
  • It also publishes a new file alongside our bundler plugins rum-browser-sdk.js which is used for the injection.

TODO

• Test RUM's SDK in real life situation.