support custom weights for svc backend server groups

#### What would you like to be added:
Kubernetes Service should support configuring weights for different backend server groups/target groups under the same Service, in order to control the traffic distribution ratio.

#### Why is this needed:
In production, there are many scenarios that require group-based traffic control, but Kubernetes Service lacks a standardized way to express weights. As a result, teams have to rely on vendor-specific features, manual configuration, or additional traffic gateways, which increases complexity and operational costs.
Typical scenarios include:
Uneven capacity across multiple availability zones (AZs): Some AZs may be capacity-constrained or more expensive, so it’s necessary to reduce the share of traffic sent to those AZs.
Disaster recovery / emergency degradation: When a backend group experiences failures or performance degradation, operators need to quickly lower its weight rather than fully removing it.

Start by reviewing how this cloud provider handles Kubernetes Services and backend server or target groups; the issue does not name specific files, entry points, or tests. First determine whether a KEP or SIG design discussion is required, then define how weights would be configured, applied, and validated before implementation. Done should include an agreed design and corresponding behavior and tests.

Written by the indexing model from the issue text.