add merge strategy to merge states during server side apply

[AmitKumarDas]

ProblemStatement: There are kubernetes resources whose specs may consist of fields with type []map[string]interface{}. This is commonly known as list map in Kubernetes world. The current server side apply logic present in metacontroller merges such types based on known merge fields. However, there are cases when known merge fields is not enough. This issue will track the required enhancement to provide custom merge fields that can be used during server side apply.

Take for example following yaml specs that does not merged by metacontroller's current logic. The entire list is treated as an atomic value and is simply replaced with the desired list during a server side apply.

kind: MyResource
spec:
  options:
  - deviceName: abc
    deviceID: abc-101
  - deviceName: def
    deviceID: abc-102

Refer:

• https://github.com/AmitKumarDas/metac/blob/master/dynamic/apply/apply.go#L361