Get valid frames from `tf2_ros.Buffer` based on `cache_time`

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There is not a method to clear the "expired" data. Due to there not being a clear definition of "now" at the library level the data is not evaluated compared to "now" but is compared to the lastest information heard on that topic. The cache expiration is only evaluated on insertion into that specific transform's history. This means that you'll have cache_time amount of history for each frame.

If you want to get the list of only frameids that have an updated data within the last X seconds I think that parsing the yaml as you're saying is likely going to be your best approach. Worrying about the precision for this seems to be a distraction compared to any useful cache duration.

A new clear method could be added to the core, however you're getting into more of the internal buffer and cache implementation and away from the specifically simple public interface for running queries.