Adds target approvals buffering below a threshold (#5168)

This PR adds target stake updates buffering to `stake-tracker`. It
exposes a configuration that defines a threshold below which the target
score *should not* be updated automatically in the target list. The
`Config::ScoreStrictUpdateThreshold` defines said threshold. If a target
stake update is below the threshold, the stake update is buffered in the
`UnsettledTargetScore` storage map while the target list is not
affected. Multiple approvals updates can be buffered for the same
target. Calling `Call::settle` will setttle the buffered approvals tally
for a given target. Setting `Config::ScoreStrictUpdateThreshold` to
`None` disables the stake approvals buffering.

- [x] benchmarks
- [x] `try-state` checks considering the unsettled score
- [x] docs

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Co-authored-by: command-bot <>