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/IntegrationTests/DownloadTests.cs

@@ -72,7 +72,8 @@ public abstract class DownloadTestBase : FunctionTestBase {
     [Fact]
     public async Async.Task Download_RedirectsToResult_WithLocationHeader() {
         // set up a file to download
-        var container = GetContainerClient("xxx");
+        var containerName = Container.Parse("xxx");
+        var container = GetContainerClient(containerName);
         await container.CreateAsync();
         await container.UploadBlobAsync("yyy", new BinaryData("content"));