fix(bigtable): Make RPC timeout configurable

[jan-auer]

Objectstore currently allows each Bigtable RPC attempt to run for 10 seconds. With two retries, this can exceed Envoy’s 15-second per-try timeout.

Add a configurable Bigtable rpc_timeout with a default of 2 seconds. The timeout is intentionally short so stalled RPCs fail faster and Objectstore can retry within Envoy’s upstream timeout budget.

getsentry/ops#23284 makes the corresponding Envoy route timeout budgets explicit.

Ref FS-502

[linear-code]

FS-502

[lcian]

Good change.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.77%. Comparing base (d25b10b) to head (d7ffa19).

Files with missing lines	Patch %	Lines
bigtable-bench/src/main.rs	0.00%	1 Missing ⚠️
objectstore-service/src/backend/bigtable.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
- Coverage   88.77%   88.77%   -0.01%     
==========================================
  Files         108      108              
  Lines       18116    18126      +10     
==========================================
+ Hits        16083    16091       +8     
- Misses       2033     2035       +2     

Components	Coverage Δ
Rust Backend	92.59% <90.90%> (+<0.01%)	⬆️
Rust Client	81.97% <ø> (ø)
Python Client	93.56% <ø> (ø)

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