More clearly define how seller trusted signals fetching works (#1230)

* Specify support for cross-origin trusted signals,

and the bundled queryFeatureSupport('*'). This also includes
fixes to some bugs in parsing of these URLs (some of the
checks were missing).

* Simpler part of feedback

* Refactor trustedBiddingSignalsURL config.

* Update scoring, also remove some diff noise.

* Split function.

* Apply feedback

* Apply feedback.

* Apply suggestions I didn't need to ask follow ups on

* Apply a typo fix Qingxin pointed out which I missed because
there was another comment about the same line.

* Fix same origin checks

* varify ignored placeholders

* Rework the merging language.

* Some first steps towards expression some fetch parallelism.

* Very vague spec of proper batcher.

* Fill out the algorithm and wire it through

* Improve some phrasing as suggested

* Fix wrong indentation level, now I can see in a preview diff.

* Fix missing comma.

* Apply feedback

* Better wait.

* Fix merge mistake

* Apply feedback

* Fix return capitalization, as per feedback

* Apply feedback

* Notes changes

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Co-authored-by: Maks Orlovich <morlovich@chromium.org>