Use default migration function if no explicit mapping override

[DanCatchpole]

Jira link (if applicable)

TBC

Change description

• If we don't define a migrationId in our mapping, then use the default pass-through to fpl-case-service behaviour.
• This will save a PR to this repository every time we want to run a default migration.
• Can override this behaviour by adding a new function in.
• Can enable STRICT validation checking for safety reasons (out of hours migrations/large migrations?) using the case-migration.validateMigrationId configuration.