Extend use of validated string types (#2357)

In the Python code there were more validated string types that haven't been properly ported for use in the C# code (such as Region). Add that type back in and improve some others:

- Use `Region` type to represent regions (implicitly convertible to/from the `AzureLocation` SDK type)
- Improve validation of `Container` type to match Azure specs and use it in more places
- Restore/fix validation of `PoolName` type which was previously removed (#2080) due to being too strict: now allows 1-64 ASCII alphanumeric/hyphen/dash
  - We want to restrict pool names so that we can use them as disambiguating prefixes for scaleset names (see #2189). Note that underscore is not actually permitted in scaleset names so we will probably end up mapping it to hyphen.

Note that once C#7 lands we will be able to simplify the usage of `ValidatedString` a lot (using static abstract methods).

----

Open questions:

For deserializing from "known-good" places such as table storage or from Azure SDK APIs, should we have an `T.UnsafeAssumeValid(string input)` method which does no validation, to protect us from breakage?

src/ApiService/Tests/SchedulerTests.cs

@@ -25,7 +25,9 @@ public class SchedulerTests {
                         TargetEnv: new Dictionary<string, string>(),
                         TargetOptions: new List<string>()),
                     Pool: new TaskPool(1, PoolName.Parse("pool")),
-                    Containers: new List<TaskContainers> { new TaskContainers(ContainerType.Setup, new Container("setup")) },
+                    Containers: new List<TaskContainers> {
+                        new TaskContainers(ContainerType.Setup, Container.Parse("setup"))
+                    },
                     Colocate: true
 
                 ),
@@ -105,7 +107,7 @@ public class SchedulerTests {
         var tasks = BuildTasks(100).Select((task, i) => {
             var containers = new List<TaskContainers>(task.Config.Containers!);
             if (i % 4 == 0) {
-                containers[0] = containers[0] with { Name = new Container("setup2") };
+                containers[0] = containers[0] with { Name = Container.Parse("setup2") };
             }
             return task with {
                 JobId = i % 2 == 0 ? jobId : task.JobId,