Strategies don't allow preferring certain submissions over others except for their age

https://github.com/channable/opsqueue/blob/78f1cc11fe0905d332a3f2fb47ef29f88afa034f/libs/opsqueue_python/src/common.rs#L102-L115

`Highest(meta_key)`/`Lowest(meta_key)`: Would make sense as this allows eating from one side of the key space

* We already have `Oldest`/`Newest`: effectively `Highest`/`Lowest` with `submission_time` as `meta_key`
* Priority based scheduling could be implemented using these strategies
* Underlying strategies could be used to move the needle further
* For high priority submissions you would prefer newest if latency is important
* For low priority submission you could prefer oldest, i.e. preferring finishing submitted work

Read libs/opsqueue_python/src/common.rs around lines 102-115, starting with the existing Oldest and Newest strategies. Trace how strategy metadata keys select submissions and define Highest(meta_key) and Lowest(meta_key) consistently with those strategies. Done means callers can select submissions by an arbitrary metadata key while retaining the existing age-based behavior.

Written by the indexing model from the issue text.