feat: support multi server names

[xuqingyun]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds support for configuring multiple domains for a single upstream cluster in KubeGateway. It allows users to define multiple domain names that route to the same backend cluster, enabling flexible deployment scenarios (e.g., internal vs. external access, blue-green deployments).

• Key Features:

  • Multi-Domain Configuration: Define multiple domains per upstream cluster in the gateway configuration.
  • Domain-Specific Routing: Maintain separate routing rules and policies for each domain.
  • Shared Routing and Rate Limiting: All domains associated with a cluster share the same routing rules and rate limit configurations, ensuring consistent behavior across access paths.
  • Domain Uniqueness Validation: Introduce checks to prevent domain name conflicts—ensuring a domain cannot be assigned to multiple clusters, which avoids routing ambiguity.

• Implementation Details:

  • Added a new serverNames field to the upstream cluster configuration schema.
  • Enhanced routing logic to handle multiple domains per cluster.

Which issue(s) this PR fixes:

Fixes #75
Also fixed #61

Special notes for your reviewer:

Does this PR introduce a user-facing change?: