Allow admin to change Proxy Configuration

**What would you like to be added**:
Changeable Proxy Configuration in Cluster CRD(clusters.anywhere.eks.amazonaws.com)

**Why is this needed**:
There is no way to change Proxy Configuration in Cluster CRD(Immutable).
Now, If there is any problem with existing Proxy, I have to recreate cluster.

Start by locating the Cluster CRD definition and the code that handles its proxy configuration and immutability. Trace how a cluster update is reconciled, then determine what must change so an administrator can update the proxy without recreating the cluster. Done means the configuration is changeable through the Cluster CRD and the update path is covered by the repository’s relevant tests.

Written by the indexing model from the issue text.