Automatically generate version suffixes for private releases of DTFx.AS and DTFx.Core

[davidmrdavid]

When making a private extension release, we usually add a version suffix to the package so that it can be distinguished from an official release.

This PR automates that process so that the CI automatically adds a version suffix when the release pipeline is triggered in a feature branch. The version suffix will be -preview.<buildVersion> where <buildVersion> is automatically generated by the CI.

Since multiple groups work in this repo, I've only made this change for DTFx.AS and DTFx.Core. In order to achieve that, I created a copy of the release pipeline yaml and filtered it to only apply to those two packages.

[jviau]

A lot of the MSBuild actions you are performing here are already part of the dotnet SDK. All we need is to set VersionPrefix and VersionSuffix. The SDK targets will handle concatenation (including the hyphen separator). I also recommend doing all the custom CI versioning tactics in MSBuild itself - I don't like relying on special CI scripts much.

You can see what I will be building towards for the Function's repos here: https://github.com/jviau/dotnet-worker-layout/tree/main/eng/targets in the Release.* files.