feat: add txpool_* related RPC methods

[natanasow]

Description

The transaction pool (txpool) is where pending transactions reside before being submitted to a node. Current APIs require full-pool scans or repeated RPC calls to track transactions, creating unnecessary overhead and inconsistent results.

Adding txpool_* methods will improve observability, developer usability, and operational efficiency. Richer structured data, better filtering, and optimized access patterns will enable faster, more reliable insights into transaction states and mempool health.

Tasks:

• Implement txpool_content
• Implement txpool_contentFrom
• Implement txpool_status

PS: more info can be found in the epic #4488 and in the design doc.

Related issue(s)

Fixes #4488

Testing Guide

Changes from original design (optional)

N/A

Additional work needed (optional)

N/A

Checklist

• I've assigned an assignee to this PR and related issue(s) (if applicable)
• I've assigned a label to this PR and related issue(s) (if applicable)
• I've assigned a milestone to this PR and related issue(s) (if applicable)
• I've updated documentation (code comments, README, etc. if applicable)
• I've done sufficient testing (unit, integration, etc.)

The base branch was changed.

[github-actions]

Test Results

 20 files  ± 0  272 suites  +3   21m 37s ⏱️ -1s
791 tests +13  787 ✅ +14  4 💤 ±0  0 ❌  - 1 
807 runs  +13  803 ✅ +14  4 💤 ±0  0 ❌  - 1 

Results for commit fbff203. ± Comparison against base commit 7bf6405.

♻️ This comment has been updated with latest results.

[quiet-node]

LGTM ready to approve after unused imports removal

[quiet-node]

LGTM! Great work as always!

[codecov]

Codecov Report

❌ Patch coverage is 99.56897% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
packages/relay/src/lib/txpool.ts	99.50%	0 Missing and 1 partial ⚠️

@@           Coverage Diff            @@
##             main    #4544    +/-   ##
========================================
  Coverage   95.41%   95.42%            
========================================
  Files         128      129     +1     
  Lines       20658    20888   +230     
  Branches     1762     1773    +11     
========================================
+ Hits        19711    19932   +221     
- Misses        928      936     +8     
- Partials       19       20     +1     

Flag	Coverage Δ
config-service	98.82% <100.00%> (+<0.01%)	⬆️
relay	90.91% <95.41%> (+0.05%)	⬆️
server	88.85% <ø> (ø)
ws-server	98.04% <ø> (ø)

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

Files with missing lines	Coverage Δ
...ckages/config-service/src/services/globalConfig.ts	100.00% <100.00%> (ø)
packages/relay/src/index.ts	100.00% <100.00%> (ø)
packages/relay/src/lib/relay.ts	98.64% <100.00%> (+0.04%)	⬆️
packages/relay/src/lib/txpool.ts	99.50% <99.50%> (ø)

... and 1 file with indirect coverage changes

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