[C++] Use S3 conditional writes to avoid some explicit checks

[pitrou]

Describe the enhancement requested

We should investigate whether S3 conditional writes can be used to reduce some of the checks we do when creating files or pseudo-directories.

We'll also have to be careful to remain compatible with non-AWS S3 implementations such as Minio, as well. This might complicate testing.

Component(s)

C++

[pitrou]

This was initially suggested by @felipecrv in #44225 (comment)