Improve Help customization

Current version: 2.0.0-beta4.23307.1 of S.CL

# Description

I wanted to customize my help

* I wanted options/aliases sorted by length, so it always shows `-s, --sl, --something-long`
* I want my options to be sorted (I can do this with `CustomizeSymbol`
* Add additional help text based on validators (I haven't found a good solution for this).
* I have validators that check for mutually exclusive options and wanted to call out the options in the help
* I have options that can mutate their required state, e.g., if you have option A and B, then C becomes required.

For sorting the options, all I really needed was a way to change the `OptionSeciton` https://github.com/dotnet/command-line-api/blob/f6ff2f1a94a58617e744f8fe0d469a2a420a6259/src/System.CommandLine/Help/HelpBuilder.Default.cs#L240

to

```csharp
ctx.HelpBuilder.WriteColumns(optionRows.OrderBy(o=>o.FirstColumnText).ToList(), ctx);
```

To do this I had to
* copy the `OptionSection` code,
* Workaround the accessibility of `ParentNode` (it's internal) and I'm not sure how safe my workaround is.
```csharp
CliCommand parentCommand = null;
var parent = current.Parents.FirstOrDefault();
```
* Implement my own version of `WriteHeading` (it's marked as private)

Are there any plans on improving S.CL help customization or expose more extension points?