Add `PagesDeployment` upload_ready field

What does this MR do and why?

Add PagesDeployment upload_ready field

Before serving pages directly from pages_deployments, we were creating an PagesDeployment and in the commit callback, the ZIP file was uploaded. Only after the PagesDeployment entry was committed to the database, we would update the ProjectPagesMetadatum to indicate the deployment was ready.

Now, serving pages directly from pages_deployments, created a problem where we're starting to serve a PagesDeployment before its upload is ready.

To fix that, a new upload_ready column will be introduced, to signalize when the new PagesDeployment upload is finished and we start to serve it.

PS.: This fix intermittent errors (404 not found) on large GitLab Pages sites, that might take longer to upload to the object storage.

Migrations

bundle exec rails db:migrate:up:main VERSION=20231115091119

main: == [advisory_lock_connection] object_id: 184260, pg_backend_pid: 55977
main: == 20231115091119 AddUploadFinishedToPagesDeployments: migrating ==============
main: -- add_column(:pages_deployments, :upload_ready, :boolean, {:default=>true})
main:    -> 0.0045s
main: == 20231115091119 AddUploadFinishedToPagesDeployments: migrated (0.0186s) =====

main: == [advisory_lock_connection] object_id: 184260, pg_backend_pid: 55977

bundle exec rails db:migrate:up:ci VERSION=20231115091119

ci: == [advisory_lock_connection] object_id: 224360, pg_backend_pid: 57557
ci: == 20231115091119 AddUploadFinishedToPagesDeployments: migrating ==============
ci: -- add_column(:pages_deployments, :upload_ready, :boolean, {:default=>true})
ci:    -> 0.0040s
ci: == 20231115091119 AddUploadFinishedToPagesDeployments: migrated (0.0354s) =====

ci: == [advisory_lock_connection] object_id: 224360, pg_backend_pid: 57557

bundle exec rails db:migrate VERSION="20231115151449"

main: == [advisory_lock_connection] object_id: 184580, pg_backend_pid: 60858
main: == 20231115151449 UpdatePagesDeploymentsUploadReadyDefaultValue: migrating ====
main: -- change_column_default(:pages_deployments, :upload_ready, {:from=>true, :to=>false})
main:    -> 0.1482s
main: == 20231115151449 UpdatePagesDeploymentsUploadReadyDefaultValue: migrated (0.1667s)

main: == [advisory_lock_connection] object_id: 184580, pg_backend_pid: 60858
ci: == [advisory_lock_connection] object_id: 184780, pg_backend_pid: 60861
ci: == 20231115151449 UpdatePagesDeploymentsUploadReadyDefaultValue: migrating ====
ci: -- change_column_default(:pages_deployments, :upload_ready, {:from=>true, :to=>false})
ci:    -> 0.0108s
ci: == 20231115151449 UpdatePagesDeploymentsUploadReadyDefaultValue: migrated (0.0496s)

ci: == [advisory_lock_connection] object_id: 184780, pg_backend_pid: 60861

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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