Implement advanced Beacon node load test command

[boulder225]

Create a more sophisticated load test command than Implement Beacon Node Tests#2998
for Beacon nodes that simulates real-world conditions and helps node operators determine their setup's capacity.

• Replace simple ping test with comprehensive API requests
• Simulate varying validator loads
• Provide actionable performance data

Acceptance Criteria:

• New load test command implemented behind a flag
• Test makes REST requests to actual CL API endpoints:
  • Get validators
  • Get blocks
  • Get attestations
  • Request proposals for current epoch
  • Request sync committee messages
• Uses real, active validator public keys from the network
• Allows specifying different numbers of simulated validators as a ceiling (e.g., 10, 100, 300)
• Increases load (simulated validators) until performance degrades
• Outputs clear performance metrics
• Documentation updated to explain new command and interpret results

[KaloyanTanev]

Done in #3361