feat: gas manager for loadtest; add tx-gas-chart

[tclemos]

closes https://github.com/0xPolygon/devtools/issues/444

This PR adds a Gas Manager to the loadtest command and introduces a new tx-gas-chart visualization tool.

Gas Manager features:

• Gas Vault: Budget-based throttling that controls transaction flow per block
• Wave Patterns: Five oscillation patterns (flat, sine, square, triangle, sawtooth) to simulate varying network conditions
• Gas Pricing Strategies: Three strategies (estimated, fixed, dynamic) for flexible gas price simulation

New tx-gas-chart command:

• Generates visual PNG charts analyzing transaction gas prices and limits across block ranges
• Displays four metrics: transaction gas prices, transaction gas limits, block gas limits, and block gas used
• Supports highlighting transactions from/to specific addresses
• Configurable scale (logarithmic or linear) for gas price visualization
• Useful for analyzing gas patterns during load tests or investigating network behavior

Use cases:

• Simulate realistic network congestion patterns during load tests
• Run infinite load tests with controlled gas consumption
• Visualize gas price and usage patterns across blocks
• Stress test networks with oscillating transaction loads