Add listener rules, modify listener, and LB attributes to load balancer service

[NitinKumar004]

Summary

• Adds 6 new types: RuleCondition, RuleAction, RuleConfig, RuleInfo, ModifyListenerInput, LBAttributes
• Adds 6 new methods to LoadBalancer interface: CreateRule, DeleteRule, DescribeRules, ModifyListener, GetLBAttributes, PutLBAttributes
• Implements across all 3 providers: AWS ELB, Azure LB, GCP LB
• Wires through portable API layer with metrics, recording, rate limiting, and error injection
• Rules support path-pattern and host-header conditions with priority ordering
• LB attributes include idle timeout, deletion protection, and access logs

Closes #63

Test plan

Unit tests (per provider)

• TestCreateRule, TestDeleteRule, TestDescribeRules
• TestModifyListener
• TestLBAttributes

Integration tests (cloudemu_test.go)

• TestListenerRulesAWS — create LB, listener, rules with path conditions
• TestModifyListenerAWS — modify listener port
• TestLBAttributesAWS — put and get attributes
• TestListenerRulesAzure, TestListenerRulesGCP

Verification

• Linter: 0 issues
• Full test suite: all passing