[FLEDGE]: Add trustedBiddingSignalsSlotSizeMode to InterestGroups.

This CL adds the trustedBiddingSignalsSlotSizeMode to the Protected
Audience API's InterestGroups, and saves them to the database. The
new field currently doesn't have any actual effect.

See WICG/turtledove#928 for API description.

Bug: 1506238
Change-Id: I7133436c813076274822db522ece1a668dba51e7

fledge/tentative/join-leave-ad-interest-group.https.window.js

@@ -481,14 +481,14 @@ const SIMPLE_JOIN_LEAVE_TEST_CASES = [
   { expectJoinSucces: true,
     expectLeaveSucces: true,
     interestGroup: { ...BASE_INTEREST_GROUP,
-      name: 'a'.repeat(1048524)
+      name: 'a'.repeat(1048520)
     },
     testCaseName: "Largest possible interest group dictionary",
   },
   { expectJoinSucces: false,
     expectLeaveSucces: true,
     interestGroup: { ...BASE_INTEREST_GROUP,
-      name: 'a'.repeat(1048525)
+      name: 'a'.repeat(1048521)
     },
     testCaseName: "Oversized interest group dictionary",
   },