feat: add domain-specific RPC method mappings

[pythonberg1997]

Summary

This PR introduces domain-based routing capabilities, allowing different domains to use different RPC method mappings to backend groups.

Changes

• Added domain_rpc_method_mappings configuration option to route requests to different backend groups based on the domain name from X-Forwarded-Host header
• Extended Config struct with DomainRPCMethodMappings field

Config example

# Default mappings (fallback)
[rpc_method_mappings]
eth_blockNumber = "query"
eth_call = "query"

# Domain-specific mappings
[domain_rpc_method_mappings."domain1.example.com"]
eth_blockNumber = "special_backend"
eth_call = "special_backend"