Add recency signal to generateBid()

FLEDGE.md

@@ -557,12 +557,13 @@ The arguments to `generateBid()` are:
 *   auctionSignals: As provided by the seller in the call to `runAdAuction()`.  This is the opportunity for the seller to provide information about the page context (ad size, publisher ID, etc), the type of auction (first-price vs second-price), and so on.
 *   perBuyerSignals: The value for _this specific buyer_ as taken from the auction config passed to `runAdAuction()`.  This can include contextual signals about the page that come from the buyer's server, if the seller is an SSP which performs a real-time bidding call to buyer servers and pipes the response back, or if the publisher page contacts the buyer's server directly.  If so, the buyer may wish to check a cryptographic signature of those signals inside `generateBid()` as protection against tampering.
 *   trustedBiddingSignals: An object whose keys are the `trustedBiddingSignalsKeys` for the interest group, and whose values are those returned in the `trustedBiddingSignals` request.
-*   browserSignals: An object constructed by the browser, containing information that the browser knows, and which the buyer's auction script might want to use or verify.  The `dataVersion` field will only be present if the `Data-Version` header was provided and had a consistent value for all of the trusted bidding signals server responses used to construct the trustedBiddingSignals. `topLevelSeller` is only present if `generateBid()` is running as part of a component auction. Additional fields can include information about both the context (e.g. the true hostname of the current page, which the seller could otherwise lie about) and about the interest group itself (e.g. times when it previously won the auction, to allow on-device frequency capping). Note that unlike for `reportWin()` the `joinCount` in `generateBid()`'s browser signals *isn't* subject to the [noising and bucketing scheme](#521-noised-and-bucketed-signals).
+*   browserSignals: An object constructed by the browser, containing information that the browser knows, and which the buyer's auction script might want to use or verify.  The `dataVersion` field will only be present if the `Data-Version` header was provided and had a consistent value for all of the trusted bidding signals server responses used to construct the trustedBiddingSignals. `topLevelSeller` is only present if `generateBid()` is running as part of a component auction. Additional fields can include information about both the context (e.g. the true hostname of the current page, which the seller could otherwise lie about) and about the interest group itself (e.g. times when it previously won the auction, to allow on-device frequency capping). Note that unlike for `reportWin()` the `joinCount` and `recency` in `generateBid()`'s browser signals *isn't* subject to the [noising and bucketing scheme](#521-noised-and-bucketed-signals). Furthermore, `recency` in `generateBid()`'s browser signals is specified in milliseconds, rounded to the nearest 100 milliseconds.
     ```
     { 'topWindowHostname': 'www.example-publisher.com',
       'seller': 'https://www.example-ssp.com',
       'topLevelSeller': 'https://www.another-ssp.com',
       'joinCount': 3,
+      'recency': 3600000,
       'bidCount': 17,
       'prevWins': [[time1,ad1],[time2,ad2],...],
       'wasmHelper': ... /* a WebAssembly.Module object based on interest group's biddingWasmHelperURL */