[explainer] Add interestGroups() method to shared storage worklet

An interestGroups() method is added to the shared storage worklet, to return the Protected Audience interest groups associated with the shared storage origin's owner, with some additional metadata.

This API provides the Protected Audience buyer with a better picture of what’s happening with their users, allowing for Private Aggregation reports (WICG/turtledove#1190).

README.md

@@ -191,6 +191,20 @@ The shared storage worklet invocation methods (`addModule`, `run`, and `selectUR
 *  `sharedStorage.context`
     *   From inside a worklet created inside a [fenced frame](https://github.com/wicg/fenced-frame/), returns a string of contextual information, if any, that the embedder had written to the [fenced frame](https://github.com/wicg/fenced-frame/)'s [FencedFrameConfig](https://github.com/WICG/fenced-frame/blob/master/explainer/fenced_frame_config.md) before the [fenced frame](https://github.com/wicg/fenced-frame/)'s navigation.
     *   If no contextual information string had been written for the given frame, returns undefined.
+*   `interestGroups()`
+    *   Returns a promise that resolves into an array of `StorageInterestGroup`. A `StorageInterestGroup` is a dictionary that extends the [AuctionAdInterestGroup](https://wicg.github.io/turtledove/#dictdef-auctionadinterestgroup) dictionary with the following attributes:
+        *   unsigned long long `joinCount`
+        *   unsigned long long `bidCount`
+        *   sequence<[PreviousWin](https://wicg.github.io/turtledove/#typedefdef-previouswin)> `previousWins`
+        *   USVString `joiningOrigin`
+        *   double `timeSinceGroupJoinedMs`
+        *   double `lifetimeRemainingMs`
+        *   double `timeSinceLastUpdateMs`
+        *   double `timeUntilNextUpdateMs`
+        *   unsigned long long `estimatedSize`
+            *   The approximate size of the contents of this interest group, in bytes.
+        *   The [AuctionAdInterestGroup](https://wicg.github.io/turtledove/#dictdef-auctionadinterestgroup)'s [lifetimeMs](https://wicg.github.io/turtledove/#dom-auctionadinterestgroup-lifetimems) field will remain unset. It's no longer applicable at query time and is replaced with attributes `timeSinceGroupJoinedMs` and `lifetimeRemainingMs`.
+        *   This API provides the Protected Audience buyer with a better picture of what's happening with their users, allowing for Private Aggregation reports.
 *   Functions exposed by the [Private Aggregation API](https://github.com/alexmturner/private-aggregation-api), e.g. `privateAggregation.contributeToHistogram()`.
     *   These functions construct and then send an aggregatable report for the private, secure [aggregation service](https://github.com/WICG/conversion-measurement-api/blob/main/AGGREGATION_SERVICE_TEE.md).
     *   The report contents (e.g. key, value) are encrypted and sent after a delay. The report can only be read by the service and processed into aggregate statistics.