Support appending to icechunk store

[abarciauskas-bgse]

As a user of icechunk's virtual references I want to be able to append to an existing icechunk store.

I made this as distinct from #21 as that issue is for appending to kerchunk-backed reference stores.

[TomNicholas]

Just to note that the reason this is still open is that we don't have any mention of it in the usage docs.

[abarciauskas-bgse]

I will go ahead and add an example to https://virtualizarr.readthedocs.io/en/latest/usage.html#writing-to-an-icechunk-store

[abarciauskas-bgse]

Closing via #272 and #409