refactor: remove vertex children from metadata

[glevco]

Motivation

This PR migrates vertex children from inside the metadata to a new standalone service, leveraging RocksDB keys to store the set of children for each vertex.

Acceptance Criteria

• Remove children from TransactionMetadata and move it to a new VertexChildrenService. A BaseTransaction.get_children() method is provided for easy compatibility.
• Remove children from reliable integration events.
• Limit graphviz endpoint to display at most 100 children per node.
• Limit children on TransactionMetadata.to_json() to contain at most 100 children. This key is kept for compatibility with APIs.
• Add migration to move vertex children to new structure.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[github-actions]

Bencher Report

Branch	refactor/vertex-children
Testbed	ubuntu-22.04

Click to view all benchmark results

Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	1.69 m (-2.58%) Baseline: 1.73 m	1.56 m (92.39%)	2.08 m (81.18%)

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[codecov]

Codecov Report

❌ Patch coverage is 80.34682% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.10%. Comparing base (8714656) to head (9f37644).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
hathor/transaction/storage/rocksdb_storage.py	18.75%	26 Missing ⚠️
hathor/graphviz.py	50.00%	5 Missing ⚠️
hathor/transaction/storage/cache_storage.py	80.00%	1 Missing ⚠️
...tion/storage/migrations/migrate_vertex_children.py	90.90%	1 Missing ⚠️
hathor/transaction/transaction.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1495      +/-   ##
==========================================
- Coverage   86.13%   86.10%   -0.03%     
==========================================
  Files         437      439       +2     
  Lines       33755    33891     +136     
  Branches     5275     5296      +21     
==========================================
+ Hits        29076    29183     +107     
- Misses       3657     3687      +30     
+ Partials     1022     1021       -1     

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