[C++] Avoid double initialisation / double finalize of Aws SDK

### Describe the usage question you have. Please include as many useful details as possible.

I have pre-existing code that uses the C++ AWS SDK and calls `Aws::InitAPI` and `Aws::ShutdownAPI`. Now I am using arrow with s3, and it seems to need me to call _arrow_'s `arrow::fs::InitializeS3()` before it will allow me to use the `S3FileSystem`. I would prefer to handle SDK initialisation myself, but I don't see a way to tell arrow that "I've already initialised the SDK, don't do it yourself". I would prefer not to add Arrow awareness to all of my existing code.

Is there a known workaround for this please?

### Component(s)

C++