Expose signing date to Javascript

While a publisher could manually incorporate the signing time into the content of their signed package, it would be easier for libraries to use that time, for example to check that content isn't "too old", if there were a standard place to look for it.

For signed exchanges, we could put it into the Document, for example document.package.signedAt. However:

1. A signed exchange can have multiple signatures with different signing times. The most important one is the one that establishes cross-origin trust, but libraries might want to be able to see other signatures.
2. We also want to be able to see the signing time for members of bundles, which might work better as a field in the Response, and then we might refer to the Response or the relevant fields from any Document initialised from it...
3. This might be a concept we should expose for other cached things, in which case the place we expose it should make sense for non-packaged things. At that point, would we just expose the Date HTTP header? The signing time(s) could be different from the Date, so we might want to expose both anyway.

Start by comparing the proposed Document and Response locations with the linked Fetch and HTML concepts. Define how multiple signature times, bundle members, and the HTTP Date value should be exposed, then document the chosen API and its behavior for packaged and non-packaged content.