Add the ability to override only a portion of the spec using Policies

[arkodg]

Description:

With policies such as the ClientTrafficPolicy, a user can target a section within a target e.g. a Listener within a Gateway which applies or overrides the target with the spec of the policy.

Another use case is where a user wants to only override some fields i.e. the top level policy might setup defaults for fields like keep alive but a specific policy for a specific target/section may want to only apply an extra setting such as retries

One way to achieve this is to add a type field within the policy resource which can be set to Replace or Merge, which defaults to Replace (apply entire spec)

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[luvk1412]

My use case where i need this:
we have a global security policy which have defined cors, each dev can select various predefined security policies for auth, cors and other stuff based on label selectors for there routes. To achieve this, merging would be required in our use case.

[arkodg]

@guicassolato and the Kuadrant team had to solve a similar problem and their solution was a strategy:Merge field
more in https://docs.kuadrant.io/0.8.0/architecture/rfcs/0009-defaults-and-overrides/#design-option-strategy-field