fix standalone SDK dependency discovery

[Shfdis]

Fix the installed standalone SDK package so it can be consumed by userver and other embedding CMake projects.

• reuse existing Protobuf, gRPC, and Abseil targets when provided by the parent project
• fall back to grpc++ pkg-config metadata when the distro gRPC package has no CMake config
• declare the missing Abseil and pkg-config Debian dependencies
• cover predefined RPC targets and the pkg-config fallback in Debian package tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.04%. Comparing base (cf7308e) to head (fa886ce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #662      +/-   ##
==========================================
+ Coverage   54.99%   55.04%   +0.04%     
==========================================
  Files         280      280              
  Lines       32702    32702              
==========================================
+ Hits        17986    18001      +15     
+ Misses      14716    14701      -15     

Flag	Coverage Δ
sdk	55.04% <ø> (+0.04%)	⬆️

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[github-actions]

🌋 SLO Test Results

🟡 2 workload(s) tested — 1 workload(s) exceeded warning thresholds

Commit: b6c114d · View run

Workload	Thresholds	Duration	Report
cpp-key-value-userver	🟢 OK	10m 2s	📄 Report
cpp-key-value	🟡 Warning	10m 4s	📄 Report

Threshold violations:

cpp-key-value:

• read_retry_attempts: Value 0.05 > warning max 0
• write_retry_attempts: ▲ 27.2% (≥ 20% warn)

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