[native] introduce addKeyserverActionType

Summary:
This diff introduces the ADD_KEYSERVER redux action type as well as the AddKeyserverPayload type that we will need for this redux action. Since at this moment we are only adding and persisting any newly added keyserver to the keyserver redux store, we don't need start, success, fail action types since we are not making any calls to an endpoint

Linear task: https://linear.app/comm/issue/ENG-5339/introduce-addkeyserveractiontype

Depends on D9590

Test Plan: flow

Reviewers: inka, michal, rohan

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9591