osd: Erasure code direct reads

[aainscow]

Current erasure code reads always go to the primary OSD. This adds latency and reduces maximum possible IOPs.

In this PR, if "replica balanced reads" are on, we direct small reads directly to the OSD containing the data and perform that read synchronously.

Early performance results are looking positive:

https://github.com/lee-j-sanders/perfresults/blob/main/cbt_24thJuly_directreads/comparitive_performance_report_250724_091106.md

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Config Diff Tool Output

+ added: osd_min_split_replica_read_size (global.yaml.in)

The above configuration changes are found in the PR. Please update the relevant release documentation if necessary.
Ignore this comment if docs are already updated. To make the "Check ceph config changes" CI check pass, please comment /config check ok and re-run the test.