[Review][C] Export the ability to get/set the log level to the C API

[ldematte]

CuVS uses raft underneath, which in turns uses rapids_logger. Users of the C++ API can access directly the logger used by raft via raft::default_logger(), and use it to get and set the log level.
This is not possible for users of the C API (and of any API based on it, like the C API).
This PR adds specific C functions to close this gap.

Closes #1310

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[cjnolet]

/ok to test 7e7d49b

[ChrisHegarty]

Do we need this in 25.10? How do we control the log level without it?

[mythrocks]

/ok to test e91a6dc

[cjnolet]

/ok to test 1959150

[copy-pr-bot]

/ok to test 1959150

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[mythrocks]

@benfred, @cjnolet: Should this change be on branch-25.12? Or is it safe to go into the current release?

I would have thought the former, given that the ability to change log levels hasn't been available in previous releases either.

But I'm Johnny-come-lately.

[cjnolet]

Unfortunately now that the code is frozen we have to push it forward, so it'll need to be in 25.12.

[mythrocks]

@cjnolet, thank you for confirming.

@ldematte: I'm 👍 on this change too. Let's retarget to branch-25.12.

[ldematte]

@mythrocks @benfred I have rebased this onto branch-25.12
I think it's ready to be merged.

[mythrocks]

/ok to test 0750e1c

[mythrocks]

Pardon me, but I don't think the changes are complete:

jextract downloaded to /__w/cuvs/cuvs/java/jextract-22
c_api.h:21:10: error: 'rapids_logger/log_levels.h' file not found
__stddef_max_align_t.h:22:15: warning: Skipping __clang_max_align_nonce2 (type LongDouble is not supported)

We'll need to change the Java build script, and change the jextract line to include the rapids_logger header directory.

[ldematte]

I have that change in #1376, along with the Java changes, but it seems the way we use jextract breaks. I'll move the change to the scripts to this PR.

[ldematte]

@mythrocks In general, I find a bit odd that when you change the C api you need to update something related to the Java API. I understand the mechanics of it (jextract process a C header which includes headers from the C API, so in turn changing a C API header may have effects), but it's counterintuitive. Imagine if I was a developer purely focusing on the C side of things, with no knowledge of jextract etc.

I think we need a more robust fix for this; I think jextract should use the same include directories as the C compiler. No idea how to achieve that though, ATM.

[mythrocks]

I think jextract should use the same include directories as the C compiler.

I agree in principle. But given that these are disparate build systems, I'm not sure how we'd unify these. :/

[mythrocks]

/ok to test 97306fe

[mythrocks]

/merge