Add VulnerabilityInfo APIs into NuGet.Protocol

[zivkan]

NuGet Product(s) Involved

NuGet SDK

The Elevator Pitch

The Vulnerabilities In Restore spec introduces a new "resource" in the V3 protocol service index document.

There needs to be APIs added in NuGet.Protocol to download them, so that not only can the rest of NuGet download and use the data, but also other apps using the NuGet.Protocol package in their own apps can use it.

Additional Context and Details

See the PR for the actual implementation, but the proposed API can be something along the lines of:

var vulnResource = await sourceRepository.GetResourceAsync<VulnerabilityInfoResource>();
IReadOnlyList<IDictionary<string, IReadOnlyList<VulnerabilityInfo>>> knownVulnerabilities =
    await vulnResource.GetVulnerabilityInfoAsync(...);

It's not the prettiest of APIs, specifically returning a list of dictionaries instead of a a single dictionary the the resource merges itself. However, it's going to be used in a perf-critical hot-path, and merging dictionaries will take more time than iterating multiple dictionaries, so here we are.