[BE] Enhancements to Subscription History Table

What does this MR do and why?

Add created_at to the LicenseType to support the Added on column in the NEW Subscription History Table.

Disclaimer: This is the backend bit that will enable the new column, there is no UI change at the Subscription History Table yet / this value is not used yet.

Screenshots or screen recordings

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Does not apply.

How to set up and validate locally

1. With your local gdk running, visit http://localhost:3000/-/graphql-explorer
2. This query should produce an output for the created_at value.

{
  currentLicense {
    id
    type
    plan
    name
    email
    company
    startsAt
    expiresAt
    createdAt
    activatedAt
    lastSync
    usersInLicenseCount
    billableUsersCount
    maximumUserCount
    usersOverLicenseCount
  }
}

# you can also just query for
{
  currentLicense {
    createdAt
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #341173 (closed)