oobmigration: derive subscription account number and extract license key fields

Created by: unknwon

This PR adds two OOB migrations:

1. Attempt to extract the account number from the associated user of subscriptions, sister PR: https://github.com/sourcegraph/sourcegraph/pull/39209
2. Extract license fields from the license key text and store them to dedicated columns, sister PR: https://github.com/sourcegraph/sourcegraph/pull/39215

Test plan

Manually e2e tested locally.

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Part of https://github.com/sourcegraph/sourcegraph/issues/38661