Support configuring jihulab.com & jihulab.hk

What does this MR do and why?

Related Issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/2425

1. 支持新的环境变量字段 SAAS_ENVIRONMENT，根据该字段判断是否为 HK 的 SaaS 环境
2. 增加 HK 环境的新域名

CN	HK
https://jihulab.com	https://jihulab.hk
https://staging.jihulab.com	https://staging.jihulab.hk
https://next.jihulab.com	https://next.jihulab.hk
https://customers.jihulab.com	https://customers.jihulab.hk
https://customers.stg.jihulab.com	https://customers.stg.jihulab.hk

How to set up and validate locally

$ bin/rails console
[1] pry(main)> Gitlab::Saas.com_url
=> "https://jihulab.com"
[2] pry(main)> Gitlab::Saas.staging_com_url
=> "https://staging.jihulab.com"
[3] pry(main)> Gitlab::Saas.canary_toggle_com_url
=> "https://next.jihulab.com"
[4] pry(main)> Gitlab::SubscriptionPortal.default_subscriptions_url
=> "https://customers.stg.jihulab.com"

$ SAAS_ENVIRONMENT=hk bin/rails console
[1] pry(main)> Gitlab::Saas.com_url
=> "https://jihulab.hk"
[2] pry(main)> Gitlab::Saas.staging_com_url
=> "https://staging.jihulab.hk"
[3] pry(main)> Gitlab::Saas.canary_toggle_com_url
=> "https://next.jihulab.hk"
[4] pry(main)> Gitlab::SubscriptionPortal.default_subscriptions_url
=> "https://customers.stg.jihulab.hk"

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cc @mtan-gitlab-cn @icbd