Generate oauth token on subscription create

What does this MR do?

For https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/3200

Dependent on https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/3355

Customers that are created as part of the subscription flow should get OAuth tokens so we can authorize resources from customers-dot. This makes it so we generate an OAuth token and send it to customers-dot on customer creation.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
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