Consider advertising implementation-defined parameters in JS and HTTP

[apasel422]

Originally posted by @apasel422 in #280 (comment)

The specification relies on a number of implementation-defined parameters, the most notable of which is the set of allowed aggregation services. That set can be retrieved via JavaScript (navigator.attribution.aggregationServices), but is not automatically accessible to the HTTP API being added in #280.

We could consider exposing it, and potentially other implementation-defined parameters, in HTTP, similar to Client Hints.

For example, a response could include:

Accept-CH: Sec-CH-Attribution-Aggregation-Services

and then receive in the future:

Sec-CH-Attribution-Aggregation-Services: "https://aggregator.example/dap";p=dap-15-histogram, "https://other-aggregator.example/foo";p=tee-00