Make config updating safe in all object stores

[paraseba]

GCS doesn't support put-if-match using etags. We may need to use an approach similar to branches.

This is necessary for 1.0 because the config on-disk format would have to change significantly.

[mpiannucci]

See https://cloud.google.com/storage/docs/request-preconditions#compose-preconditions for details

[rabernat]

This is confusing to me. It certainly looks like it is implemented

If-Match header	Applicable for requests that retrieve data. Request proceeds if the ETag of the target resource matches the value used in the precondition. If the values don't match, the request fails with a 412 Precondition Failed response.

[mpiannucci]

On gcs if-match is only avaialable for Get or Head requests. For puts, you have to use the generation match functionality which currently we don't have a mechanism for in icechunk. Object store supports it but you have to track the version (generation) instead of the ETag which we don't do currently

[rabernat]

So AWS S3 and GCS have mutually incompatible ways of achieving conditional put: etag (S3) vs generation number (GCS). And it's up to the application (e.g. Icechunk) to figure out which to use when.