feature request: allow override "sub" claim in ID Token claims

What problem did you meet?

• we already have users in our previous IdP (Auth0)
• we have existing database that store user_id anywhere
• we want to migrate our existing users to Logto
• our RP-app identified the user as different because we identify the user_id from "sub" claim of the ID Token or userinfo endpoint

Describe what you'd like Logto to have

can override the ID Token claim with

return { "sub": event.context.user.customData?.legacyUserId ?? event.context.user.id }`

Current Implementation

enable the custom_data claim and modify RP-app code everywhere I need the user_id based on custom_data claim and fallback to sub claim for new users

user_id = userinfo.get("custom_data", {}).get("legacyUserId") or userinfo.get("sub")