Add new table: ci_minutes_additional_packs

What does this MR do?

As part of &5715, we are implementing the foundation for a new way to provision CI minute purchases and improve the synchronization (and expiry) of those purchases between CustomersDot and GitLab.

This is step 1 of the process: adding a new table (and associated model) to track the data within GitLab. (customers-gitlab-com#2924 (closed))

For now, this new table and model will be unused.

The proposal that discusses the approach being taken here can be found documented here: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/staging/doc/ci_minutes/provisioning_proposal.md#proposal

Database info

DB migration

bin/rails db:migrate

== 20210526135911 CreateCiMinutesAdditionalPacks: migrating ===================
-- create_table(:ci_minutes_additional_packs, {:if_not_exists=>true})
-- quote_column_name(:purchase_xid)
   -> 0.0000s
   -> 0.0128s
-- quote_table_name("check_d7ef254af0")
   -> 0.0000s
-- quote_table_name(:ci_minutes_additional_packs)
   -> 0.0000s
-- execute("ALTER TABLE \"ci_minutes_additional_packs\"\nADD CONSTRAINT \"check_d7ef254af0\" CHECK (char_length(\"purchase_xid\") <= 32)\n")
   -> 0.0013s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:ci_minutes_additional_packs, [:namespace_id, :purchase_xid], {:name=>"index_ci_minutes_additional_packs_on_namespace_id_purchase_xid", :algorithm=>:concurrently})
   -> 0.0019s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:ci_minutes_additional_packs, [:namespace_id, :purchase_xid], {:name=>"index_ci_minutes_additional_packs_on_namespace_id_purchase_xid", :algorithm=>:concurrently})
   -> 0.0021s
-- execute("RESET ALL")
   -> 0.0005s
== 20210526135911 CreateCiMinutesAdditionalPacks: migrated (0.0298s) ==========

DB rollback

bin/rails db:rollback

== 20210526135911 CreateCiMinutesAdditionalPacks: reverting ===================
-- drop_table(:ci_minutes_additional_packs)
   -> 0.0046s
== 20210526135911 CreateCiMinutesAdditionalPacks: reverted (0.0171s) ==========

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• Label as security and @ mention @gitlab-com/gl-security/appsec
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