rerun_py: Improvements to dataframe and simple unit-test framework

[jleibs]

What

• Allow select() to support *args
• Return a RecordBatchReader instead of list[RecordBatch]
• Add a unit-test that creates a recording, opens it, and does some stuff.

We still aren't able to iterate over the result-set without collecting it, as we need to fix:

rerun/crates/store/re_dataframe2/src/engine.rs

Lines 31 to 34 in b5aa6e9

	// TODO(cmc): This needs to be a refcounted type that can be easily be passed around: the ref has
	// got to go. But for that we need to generally introduce `ChunkStoreHandle` and `QueryCacheHandle`
	// first, and this is not as straightforward as it seems.
	pub struct QueryEngine<'a> {

Future work:

• Lots of fun stuff to do here with the zoo

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary
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