Add namespace_ci_cd_settings table

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds a new namespace_ci_cd_settings table. This came from the need to have an indexed boolean setting assigned to namespaces which will determine which namespaces opted into stable CI runner pruning (this will be a cronjob to be introduced later on - see issue).

When tackling this, I looked at 2 options:

1. Reusing namespace_settings by creating a field and an index there.

• Advantage: Existing table, will play well when we need to extend the scope of this MVC to instance runners and project runners (with application_settings and project_settings/project_ci_cd_settings);
• Disadvantages:
  • Lots of wasted space considering that a lot of namespaces will not have custom settings;
  • Could get unwieldy as time goes on if we continue to need to add runner-related settings;

1. Creating a new namespace_ci_cd_settings table, similar to project_ci_cd_settings;

• Advantages:
  • only namespaces that opt-in will have a row in this new table;
  • easier to add new runner-related settings in the future and keep them together;
• Disadvantage:
  • We don't gain the existing hierarchy support from application_settings/namespace_settings/project_settings), although this seems to be encapsulated in a reusable module (CascadingNamespaceSettingAttribute);

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How to set up and validate locally

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Migrations

Migrate up

== 20220503102855 AddNamespaceCiCdSettingsTable: migrating ====================
-- create_table(:namespace_ci_cd_settings, {})
   -> 0.0053s
== 20220503102855 AddNamespaceCiCdSettingsTable: migrated (0.0053s) ===========

Migrate down

== 20220503102855 AddNamespaceCiCdSettingsTable: reverting ====================
-- drop_table(:namespace_ci_cd_settings, {:if_exists=>true})
   -> 0.0021s
== 20220503102855 AddNamespaceCiCdSettingsTable: reverted (0.0021s) ===========

MR acceptance checklist

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

Part of Implement worker to remove stale runners from G... (#361112 - closed)