Add build script with auto-version

Add a build script that automatically sets the version info in the .csproj file based on the git tag data (git describe).

When packaging the library the version number should be based of the git tag so that git is the system of record for version numbers.

Also include some way to identify whether the publish was done on a git tag, or not. For example, if the publish runs on a commit tagged as v0.2.0, the version should be 0.2.0, but if the publish runs on a later commit, the version should be something like 0.2.0-2a5e1 (either include the commit hash or some other identifier)