Expose unsafe / rename commit handlers (potentially through storage options?)

[westonpace]

We recently changed to a conditional put strategy by default. This is because S3 now allows conditional puts and so all major providers allow it.

However, there are some "S3-compatible" API implementations which have not yet implemented conditional put (at the moment, I'm not aware of any that are not proprietary). If we want to continue supporting these stores then we could make the strategy configurable, perhaps via storage options.

[wjones127]

I did keep the rename handler in the rust code so someone could use it. Agreed we could add something in storage options to let a user select this.