feat(cogs): publish backend change streams to Kafka

[matt-codecov]

Ref FS-210

Add CostTrackerConfig struct to objectstore-server's config. This allows storage COGS to be configured end-to-end; CostTrackerConfig on objectstore-server's config and CostTrackerStreamConfig on each Backend impl will produce a CostTrackerStream that holds a shared Kafka producer and streams change messages on the shared-resources-inventory topic.

While ChangeStream is abstract, the abstraction is not exposed in config. It's assumed each ChangeStream impl will name itself explicitly in dedicated config sections.

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[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.47%. Comparing base (985b88f) to head (f42ac54).

Files with missing lines	Patch %	Lines
objectstore-server/src/config.rs	95.00%	2 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                          @@
##           matth/storage-inventory-tracker-3     #597      +/-   ##
=====================================================================
+ Coverage                              88.39%   88.47%   +0.07%     
=====================================================================
  Files                                    104      104              
  Lines                                  17129    17173      +44     
=====================================================================
+ Hits                                   15142    15194      +52     
+ Misses                                  1987     1979       -8     

Components	Coverage Δ
Rust Backend	92.48% <95.65%> (+0.03%)	⬆️
Rust Client	81.97% <ø> (ø)
Python Client	93.38% <ø> (ø)

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[linear-code]

FS-210

[jan-auer]

nit: Reading it like this, implementing Default for ChangeStreamFactory leaves the question of what the default actually does. There's no natural "default" for change streams. Maybe we should remove that and explicitly instantiate the noop factory here.

[matt-codecov]

there isn't a separate noop factory, there's just ChangeStreamFactory. the way i think of it is: the default factory has no sink configs and only knows how to build NoopStream. if you have sink configs, you build a non-default factory with them and that factory knows how to build other ChangeStreams