Describe checkpoints in .feature files

In #6094 (closed) we introduced checkpoint descriptions in features/step_definitions/snapshots.rb. It would be nice if we could describe them in .feature files instead, e.g. in features/step_definitions/checkpoint/*.feature. Presumably we could use the correct bits of cucumber to parse those .feature files (and it’d be great if we could call them .checkpoint instead) and get the info we need.

Some details about how to deal with the metadata/fields:

• The :parent_checkpoint field is dealt with by first step, which should match the parent checkpoint’s Scenario name.
• The :temporary field is dealt with by a @temporary_checkpoint tag.

Parent Task: #10237

Original created by @anonym on 10341 (Redmine)