AlexandraZapuc

[FINAL] feat: Canister backup and restore

9

Adds new API to the management canister that facilitates backup and restore operations. Design doc: [backup and restore](https://docs.google.com/document/d/1vIZGpdYALSkeOugK9EQiaTiiAGKw8EOn__etQjGJUwM/edit) Note to reviewers: opened MR to request feedback from the interface spec...

This PR renames EnvironmentVariables to CanisterEnvironmentVariables