Proposal for how to close #1208.

With this we get

In [1]: import icechunk
   ...: 
   ...: storage = icechunk.in_memory_storage()
   ...: repo = icechunk.Repository.create(storage)

In [2]: repo.readonly_session("main")
Out[2]: 
<icechunk.Session>
read_only: True
snapshot_id: 1CECHNKREP0F1RSTCMT0

In [3]: repo.writable_session("main")
Out[3]: 
<icechunk.Session>
read_only: False
snapshot_id: 1CECHNKREP0F1RSTCMT0
branch: main
has_uncommitted_changes: False

For compound classes I want to have them make nested repr calls, i.e. Repository.__repr__ includes RepositoryConfig.__repr__ and Storage.__repr__, passed as "attributes". This way we should be able to have nice nested reprs for all classes be implemented simply and consistently. It should also be easy to change the reprs if necessary, or to match any API changes later.

Then once that's done we can use the same approach but for HTML reprs in a follow-up PR.