Merge #1830

1830: Add skeleton `retirement` field to `ApiStakePool`.  r=jonathanknowles a=jonathanknowles

# Issue Number

#1818 

# Overview

This PR:

- [x] Adds a skeleton `retirement` field to the `ApiStakePool` type:
    ```patch
    data ApiStakePool = ApiStakePool  
        { id :: !(ApiT PoolId)
        , metrics :: !ApiStakePoolMetrics 
        , metadata :: !(Maybe (ApiT StakePoolMetadata))   
        , cost :: !(Maybe (Quantity "lovelace" Natural))  
        , margin :: !(Maybe (Quantity "percent" Percentage))  
        , pledge :: !(Maybe (Quantity "lovelace" Natural))
    +   , retirement :: !(Maybe ApiEpochInfo)
        } deriving (Eq, Generic, Show)
    ````
- [x] Updates the equivalent swagger definition for `ApiStakePool`.
- [x] Updates the golden JSON samples for `ApiStakePool`.
- [x] Promotes `ApiEpochInfo` to a top-level swagger definition, making it easier to reuse.
- [x] Fixes (for the moment) the value of the `retirement` to `Nothing`. (Future PRs will arrange that this value reflects the actual retirement status of a pool.)

Co-authored-by: Jonathan Knowles <jonathan.knowles@iohk.io>

lib/core/src/Cardano/Wallet/Api/Types.hs

@@ -396,6 +396,7 @@ data ApiStakePool = ApiStakePool
     , cost :: !(Maybe (Quantity "lovelace" Natural))
     , margin :: !(Maybe (Quantity "percent" Percentage))
     , pledge :: !(Maybe (Quantity "lovelace" Natural))
+    , retirement :: !(Maybe ApiEpochInfo)
     } deriving (Eq, Generic, Show)
 
 data ApiStakePoolMetrics = ApiStakePoolMetrics