feat(gateways): SASL/PLAIN authentication for the Kafka gateway

[numinnex]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.96%. Comparing base (2894368) to head (3739da8).

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #4222       +/-   ##
=============================================
- Coverage     87.62%   26.96%   -60.66%     
  Complexity     1575     1575               
=============================================
  Files          1284     1281        -3     
  Lines        225457   181924    -43533     
  Branches     188820   145288    -43532     
=============================================
- Hits         197549    49054   -148495     
- Misses        23177   131145   +107968     
+ Partials       4731     1725     -3006     

Components	Coverage Δ
Rust Core	11.75% <ø> (-76.97%)	⬇️
Java SDK	68.68% <ø> (ø)
C# SDK	77.41% <ø> (+0.06%)	⬆️
Python SDK	90.97% <ø> (ø)
PHP SDK	85.67% <ø> (ø)
Node SDK	96.44% <ø> (+0.07%)	⬆️
Go SDK	70.14% <ø> (+0.08%)	⬆️
see 693 files with indirect coverage changes

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[hubcio]

The SASL path has one blocking issue and a few smaller ones. The blocking one is the pre-auth ApiVersions allowance: it is only spent on a success answer, so a client that repeats a rejected version holds a max_connections permit forever. The rest are a startup panic from one env var, a concurrency default above the shard count, and some doc and dead-code cleanups.

[numinnex]

/ready