Update rpc_bench to support fixed block ranges and file output

[Kbhat1]

Describe your changes and provide context

• Add explicit start/end block selection to rpc_bench
• Send output to configurable file location

Testing performed to validate your change

• Verified on node

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 23, 2026, 9:07 PM

[codecov]

Codecov Report

❌ Patch coverage is 0% with 116 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.30%. Comparing base (491d2c6) to head (8c48573).

Files with missing lines	Patch %	Lines
sei-db/tools/rpc_bench/main.go	0.00%	116 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3110      +/-   ##
==========================================
- Coverage   58.60%   58.30%   -0.31%     
==========================================
  Files        2096     2096              
  Lines      173400   173497      +97     
==========================================
- Hits       101621   101149     -472     
- Misses      62739    63291     +552     
- Partials     9040     9057      +17     

Flag	Coverage Δ
sei-chain-pr	0.00% <0.00%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-db/tools/rpc_bench/main.go	0.00% <0.00%> (ø)

... and 20 files with indirect coverage changes

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