feat: add title, summary and description to the pipeline model

[morbalint]

BIVS-1969

Checklist

• I've read and followed the Contribution Guidelines
• README.md is updated with the changes (if needed)

Version

Requires a MINOR version update

Context

More human readable description of pipelines and workflow to enable adding more visibility to them, by adding a title, summary and description field to pipeline elements. Registration of these fields is required so that the WFE workflows tab save button doesn't remove them from an existing bitrise.yml.

Changes

Adds title, summary and description to the PipelineModel and StageModel

Investigation details

When editing the bitrise.yml in the visual WFE, the save button on the workflows tab removes all unknown fields 😞

Decisions

[godrei]

Pls fill in the PR template to let others understand this change in the future.