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).
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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?
