Create "migrations/plan" endpoint for creating migration plans.

The migration planning endpoint will need to accept some data: the set
of addresses to which the wallet balance should be migrated. Without a
set of addresses, it's impossible to construct the coin selections
required for a migration plan, as every coin selection output must have
an address: these can only be provided by the caller.

However, the "migrateByronWallet" and "migrateShelleyWallet" operations
at the "migrations" endpoint already accept POST data.

We need a way to discriminate between the following operations:

1. Creating a plan.
2. Creating a plan and executing it immediately.

Assuming both types of operation require data to be POSTed, this means
we need different endpoint paths.

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

@@ -518,6 +518,7 @@ type MigrateShelleyWallet n = "wallets"
 type GetShelleyWalletMigrationInfo n = "wallets"
     :> Capture "walletId" (ApiT WalletId)
     :> "migrations"
+    :> "plan"
     :> Get '[JSON] (ApiWalletMigrationInfo n)
 
 {-------------------------------------------------------------------------------
@@ -800,6 +801,7 @@ type MigrateByronWallet n = "byron-wallets"
 type GetByronWalletMigrationInfo n = "byron-wallets"
     :> Capture "walletId" (ApiT WalletId)
     :> "migrations"
+    :> "plan"
     :> Get '[JSON] (ApiWalletMigrationInfo n)
 
 {-------------------------------------------------------------------------------

specifications/api/swagger.yaml

@@ -4601,6 +4601,7 @@ paths:
             schema: *ApiShelleyWalletMigrationPostData
       responses: *responsesMigrateWallet
 
+  /wallets/{walletId}/migrations/plan:
     get:
       operationId: getShelleyWalletMigrationInfo
       tags: ["Migrations"]
@@ -5005,6 +5006,7 @@ paths:
             schema: *ApiByronWalletMigrationPostData
       responses: *responsesMigrateWallet
 
+  /byron-wallets/{walletId}/migrations/plan:
     get:
       operationId: getByronWalletMigrationInfo
       tags: ["Byron Migrations"]