Update cost margin configuration.

Updates the cost_margin_config in litellm_settings. Margins can be:

• Percentage: {"openai": 0.10} = 10% margin
• Fixed amount: {"openai": {"fixed_amount": 0.001}} = $0.001 per request
• Combined: {"vertex_ai": {"percentage": 0.08, "fixed_amount": 0.0005}}
• Global: {"global": 0.05} = 5% global margin on all providers

Example:

{
    "global": 0.05,
    "openai": 0.10,
    "anthropic": {"fixed_amount": 0.001},
    "vertex_ai": {"percentage": 0.08, "fixed_amount": 0.0005}
}