Add example for using a separate threadpool for CPU bound work

[alamb]

TODOs:

• Add some example of trying to do IO on an object store (and should error with no io registered)
• 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 Document DataFusion Threading / tokio runtimes (how to separate IO and CPU bound work) #12393

Rationale for this change

I added documentation that explains the problem here:

• Improve documentation (and ASCII art) about streaming execution, and thread pools #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 Add DedicatedExecutor to FlightSQL Server datafusion-contrib/datafusion-dft#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?

[crepererum]

I think this is the tricky bit and shouldn't be underestimated. This means that for streaming responses you need to buffer data somewhere or accept higher latency. Also note that if you use ANY form of IO within DF (e.g. to talk to the object store) and, you need to isolate that as well.

So to me that mostly looks like a hack/workaround for DF not handling this properly by default.

[alamb]

I agree -- I hope that the different_runtime_advanced will show how do it "right" -- I haven't yet figued out how to do it.

@tustvold and @matthewmturner and I have been discussing the same issue here: datafusion-contrib/datafusion-dft#248 (comment)