Make S3Storage get_client pub

[d0rianb]

Is there any particular reason for this method to be private :

icechunk/icechunk/src/storage/s3.rs

Line 246 in 4ce8e67

async fn get_client(&self, settings: &Settings) -> &Arc<Client> {

It would be nice to be able to use it to retrieve the s3 client from the storage, to benefit from the init_or_get single initalization

[paraseba]

I'm OK with this. Would you like to provide a PR @d0rianb ?