Optimise asset-fetching in task

### Body Currently inlet resolution (in `_AssetMainOperator`), `InletEventsAccessors`, and `OutEventsAccessors` all implement their own asset resolution logic. This is not only inefficient, but also a lot of duplication. We should try to put the logic in one place and add some simple cache. ### Committer - [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.