[Raven-L7] Gateway manager optimization

[zzguang]

We need to optimize gateway manager to achieve the goal below:

1. gateway CR can be auto-created/deleted for solo edge nodes by default:
   1). For solo edge nodes, treat it as a gateway node by default and try to launch l7 proxy agent
   2). Whether to create a gateway CR to notify other gateways is TBD, it's better to make it transparent to users
2. gateway CR can be auto-created/deleted for nodepools by default:
   1). When a nodepool is created, and some nodes are added into the pool, a default gateway is created for the pool
   2). When some other nodes are added to/removed from the pool, the default gateway endpoints can be updated dynamically
   3). When the nodepool is deleted, the corresponding gateway CR is deleted automatically from the cluster
   4). When users updated a gateway of a pool, how to maintain it automatically is TBD

There are more technical details need to be nailed down for this feature, we can implement step 1 for solo edge nodes firstly at current stage, how to optimize it for nodepool usage scenarios needs more investigation.