Add implicit conversion from MinVerVersion to String

[augustoproiete]

To simplify passing the version as argument to the different tools in the build pipeline (e.g. dotnet build, dotnet pack, etc.).

var version = MinVer();

// Currently
var customVersion1 = BuildCustomVersion(version.Version);
var customVersion2 = BuildCustomVersion(version.ToString());

// Desired:
var customVersion2 = BuildCustomVersion(version);

// ---

static string BuildCustomVersion(string versionString)
{
    // ...
}