Should reports be visible to extensions? #223

As brought up in #169:

2. Will reports be exposed to webExtension APIs, for extension controlled blocking and filtering?
3. Many privacy-preserving resource block on the basis of 1p vs 3p communication. There should be someway of mirroring this information to other decision points (e.g. an extension should see both the destination of the report and the source of it, and be able to say yes / no accordingly)

In general, browser extension frameworks are outside the realm of standards: they change fast, and may be deeply tied to the internals of individual browsers, and generally operate at a different level than web APIs permit. However, https://browserext.github.io/browserext/ would suggest that there might be some common ground, and while it might not be possible to do anything normative in the Reporting spec, maybe there is a way for the privacy considerations section to suggest that outgoing reports be interceptable by an extensions framework, if one is available.