feat: add --disable-publish flag to load test commands

[enesuludg]

• Add CanPublish field to TesterParams and Params structs
• Add --disable-publish flag to both perf load-test and load-test commands
• Implement custom token generation with canPublish permission control
• Update LoadTester.Start() to use custom tokens when canPublish is false
• Maintain backward compatibility with existing behavior
• Rename conflicting newAccessToken in agentloadtester.go to newAgentAccessToken

This allows users to test scenarios where participants cannot publish tracks by setting canPublish: false in JWT tokens.