Add example for using a separate threadpool for CPU bound work

[alamb]

TODOs:

• [x] Add some example of trying to do IO on an object store (and should error with no io registered)
• [x] Contemplate adding the DedicatedExecutor code to make the example simple
• [ ] Add wrapper over object store
• [ ] Add wrapper over streams on the DedicatedExecutor
• [ ] Complete example
• [ ] Split the DedicatedExecutor / etc into its own PR

Which issue does this PR close?

• Closes https://github.com/apache/datafusion/issues/12393

Rationale for this change

I added documentation that explains the problem here:

• https://github.com/apache/datafusion/pull/13423

But now we need to show people how to fix it

I am reminded when trying to do this how non obvious it is

What changes are included in this PR?

Add a well commented example of how to use mutiple runtimes

The DedicatedExecutor code is orginally from

1. InfluxDB 3.0 (todo link), largely written by @tustvold and @crepererum
2. Largely based on https://github.com/datafusion-contrib/datafusion-dft/pull/247 from @matthewmturner

The XXX object store code is also based on work from @matthewmturner in

• https://github.com/datafusion-contrib/datafus

Are these changes tested?

By CI

Are there any user-facing changes?