[spec] Enable per-context contribution limits for Private Aggregation

The goal is to enable Shared Storage embedders to override the default
number of contributions per Private Aggregation report.

To that end, this change adds the `maxContributions` field to the
web-visible Private Aggregation config dictionary and plumbs its value
into Private Aggregation's "pre-specified report parameters".

Context:
* Explainer: patcg-individual-drafts/private-aggregation-api#146
* Spec change: patcg-individual-drafts/private-aggregation-api#164

spec.bs

@@ -575,11 +575,24 @@ Moreover, each {{SharedStorageWorklet}}'s [=global scopes|list of global scopes=
         1. If |filteringIdMaxBytes| is not the [=default filtering ID max bytes=] or
             |contextId| is not null, return a new {{DOMException}} with name
             "`DataError`".
+    1. Let |maxContributions| be null.
+    1. If
+        |privateAggregationConfig|["{{SharedStoragePrivateAggregationConfig/maxContributions}}"]
+        [=map/exists=], set |maxContributions| to
+        |privateAggregationConfig|["{{SharedStoragePrivateAggregationConfig/maxContributions}}"].
+    1. If |maxContributions| is not a positive integer, set |maxContributions| to null.
     1. Return a new [=pre-specified report parameters=] with the items:
         : <a spec="private-aggregation-api" for="pre-specified report parameters">context ID</a>
         :: |contextId|
         : [=pre-specified report parameters/filtering ID max bytes=]
         :: |filteringIdMaxBytes|
+        : <var ignore>requested max contributions</var>
+        :: |maxContributions|
+
+        Issue: Remove the `ignore` tag on <var ignore>requested max
+               contributions</var> once Private Aggregation's [PR
+               #164](https://github.com/patcg-individual-drafts/private-aggregation-api/pull/164)
+               is merged.
   </div>
 
   <div algorithm>
@@ -1446,6 +1459,7 @@ On the other hand, methods for getting data from the [=shared storage database=]
     USVString aggregationCoordinatorOrigin;
     USVString contextId;
     [EnforceRange] unsigned long long filteringIdMaxBytes;
+    [EnforceRange] unsigned long long maxContributions;
   };
 
   dictionary SharedStorageRunOperationMethodOptions {