Add a __dataframe__ method to _VaexDataFrame

[rgommers]

This will align the implementation with those in other libraries, xref data-apis/dataframe-api#80.

Cc @maartenbreddels, @honno

[rgommers]

(apologies for iterating in CI, I was a bit too lazy to set up a dev environment for these few lines of code)

[rgommers]

This seems happy now, the 4 CI failures I inspected are all HTTPClientError: HTTP 502: Bad Gateway.

[honno]

FWIW dataframe_protocol_tests.py and the relevant dunder-df tests in dataframe-interchange-tests pass for this PR locally.

(If the bad gateway stuff gets resolved, note there's other issues with interchange stuff on win/mac CI #2150 (comment))

[rgommers]

@maartenbreddels I somehow keep getting notifications for new failures on reruns of CI jobs. No worries if you are doing that on purpose - if not, something is probably off in the GitHub Actions config in this repo.

The only failures are still (7x):

FAILED tests/viz_test.py::test_heatmap_with_what[webserver_fastapi-ds_filtered-tornado_client]

[maartenbreddels]

Yeah, that was me :) I had to rebase, hopefully it is green now.

[maartenbreddels]

green enough, thanks Ralf!

[rgommers]

great, thanks!