Signing: support multiple timestamps on each signature

[dtivel]

Enable NuGet clients to support multiple timestamps on each signature. In the event that there is a problem with one timestamp, signature validation can fall back to another timestamp. This will improve long-term trust viability.

Note that NuGet client already allows multiple timestamps but only recognizes the first one during signature verification here and
here.

nuget.org requires exactly 1 timestamp on each signature, and that would need to change too.