AirbyteLib: Add len() support on SQL datasets and Mapping behaviors for ReadResult (#34763)

[aaronsteers]

1. Make ReadResult inherit from Mapping[str, CachedDataset], including support for .keys() and .len().
2. Add len() supports on datasets.

[vercel]

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[aaronsteers]

@flash1293 - This responds to some beta user feedback:

1. Intuitively expecting ReadResult to support a .keys() method. (It didn't but now it would.)
2. Responding to the weirdness of len(list(result["users"])) requiring a nested list() call. Now __len__() is implemented so that len(result["users"]) works as expected.

[flash1293]

Makes sense as convenience functions - I'm worried a little bit about len being called unexpectedly by something on a lazy dataset and taking a long time to finish, but it's probably OK as it's consistent.

[flash1293]

Actually, there are some problems with the implementation, left notes.

[aaronsteers]

Makes sense as convenience functions - I'm worried a little bit about len being called unexpectedly by something on a lazy dataset and taking a long time to finish, but it's probably OK as it's consistent.

Yeah - I ran into the same concern upfront. But on reflecting more, if anything is explicitly requesting the num items, I think its okay to count them. 🤷 There are plenty of small sources where this is a reasonable thing to do. And for larger sources, it seems the user would be aware of the issue.

The reason I'd revert is if we find there's some implicit or unexpected call to len() that the user would not anticipate.