Implement migrateStrategy

[MickdeGraaf]

User Story

• As an <audience/user>: PieVault admin
• I want to <action/task>: migrate one strategy to another
• so that <outcome/goal/benefit>: strategies can be upgraded/replaced

Type

• Enhancement
• Maintenance
• Refactor

Description

Should withdraw all assets from one strategy and migrate it to another one.

Definition of Done

• Implement migrateStrategy
• Unit test migrateStrategy

Part of #30
See Yearn implementation