Unified Backups: Introduce `backup_information.json` metadata

What does this MR do and why?

This introduces a new metadata format for the Unified Backups: backup_information.json.

The change from old file in YAML to new file being in JSON is to allow defining a JSON Schema to validate the file in a later stage.

This also makes the two backups incompatible, which removes the need to implement a different validation mechanism on the old one.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

When a Backup is performed, it should include on the root folder a file: backup_information.json with the following keys:

• metadata_version
• backup_id
• created_at
• gitlab_version

{
  "metadata_version": 2,
  "backup_id": "1712944480_2024_04_12_16.11.0-pre",
  "created_at": "2024-04-12T17:54:40Z",
  "gitlab_version": "16.11.0-pre"
}

Related to #427337 (closed)

gems/gitlab-backup-cli/lib/gitlab/backup/cli/backup_metadata.rb

@@ -3,7 +3,17 @@
 module Gitlab
   module Backup
     module Cli
+      # Backup Metadata includes information about the Backup
       class BackupMetadata
+        # Metadata version number should always increase when:
+        # - field is added
+        # - field is removed
+        # - field format/type changes
+        METADATA_VERSION = 2
+
+        # Metadata filename used when writing or loading from a filesystem location
+        METADATA_FILENAME = 'backup_information.json'
+
         # Unique ID for a backup
         # @return [String]
         attr_reader :backup_id
@@ -35,6 +45,33 @@ def self.build(gitlab_version:)
             gitlab_version: gitlab_version
           )
         end
+
+        # Expose the information that will be part of the Metadata JSON file
+        def to_hash
+          {
+            metadata_version: METADATA_VERSION,
+            backup_id: backup_id,
+            created_at: created_at.iso8601,
+            gitlab_version: gitlab_version
+          }
+        end
+
+        def write!(basepath)
+          basepath = Pathname(basepath) unless basepath.is_a? Pathname
+
+          json_file = basepath.join(METADATA_FILENAME)
+          json = JSON.pretty_generate(to_hash)
+
+          json_file.open(File::RDWR | File::CREAT, 0o644) do |file|
+            file.write(json)
+          end
+
+          true
+        rescue IOError, Errno::ENOENT => e
+          Gitlab::Backup::Cli::Output.error("Failed to Backup information to: #{json_file} (#{e.message})")
+
+          false
+        end
       end
     end
   end