Dynamic adoption of shadow mode

[wassgha]

Related to #22357 (part of #25500)

In order to offload code from extensions that use the MultiDocManager internally, one approach would be to allow adoption of shadow mode when such extensions are encountered which would expose the same API from PWAs to the extensions.

Implications

• The extensions would now be able to use AMP.attachShadowDoc from the global object as opposed to having to create their own instance of MultiDocManager).

Tasks

• Separate the overlaid shadow mode layer code from amp-shadow
• Detect extensions that require shadow mode and dynamically adopt it before loading the extensions
• Unit testing & e2e testing if needed