feature: VS drop optimization data fallback

https://github.com/dotnet/arcade/blob/main/src/Microsoft.DotNet.Arcade.Sdk/tools/VisualStudio.AcquireOptimizationData.targets

These targets specify looking for an optimization drop on the branch where we're building. This does not work on the first build of a branch and requires manual override which is painful for MSBuild that we need to remember this for a release every month.

Proposal: adjust the targets so an override fallback branch can be specified (would be added in msbuild's official build as main) then during the official build it'll search for a branch drop, and if nothing is found then search on fallback branch.