[c++] Simplify internal API for dataframe-resizer

[johnkerl]

Issue and/or context: As tracked on issue #2407 / [sc-51048].

Note that the intended Python and R API changes are all agreed on and finalized as described in #2407.

Changes:

While the full-generality upgrade_domain is tasked out on #2407, the foremost use-case for append mode, and tests/testthat/test-write-soma-resume.R for PR #3089, will need a simpller keystroke-saver which is already implemented in C++.

What this existing method does in C++ is resize the soma_joinid dim, if it's a dim, and leave the other dims as-is; if soma_joinid is not a dim, leave it as-is. This exists in C++, it's unit-tested in C++, and it's ready (and it's time) to be plumbed up to Python and R as well.

I realize now that having C++ maybe_resize_joinid take a length-1 vector, and assert that it's length-1, is silly -- it should simply take a single int64_t. This PR does just that. It also removes the confusing prefix maybe_.

Notes for Reviewer:

[codecov]

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[johnkerl]

Thanks @nguyenv !