Allow iterable datasets with no known size

Feature request

Some iterable datasets would have no pre-defined size, e.g. data is generated or augmented on the fly, or streamed in a way that the total number of samples is not known in advance. I'm facing this issue when streaming data from LSDB in the Uncle Val project, because any change to the LSDB query would lead to a different number of rows, and getting this number would basically mean streaming the whole dataset first.

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Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

No files, tests, or entry points are identified in the issue. Start by locating how Hyrax represents dataset size and iteration, then define how unknown-size datasets should behave and validate the design against the streaming LSDB use case.