MVP: Lead time for changes DORA Metric

[brunocribeiro]

Goal: The OpenDORA plugin is also capable of showing the Lead time for changes DORA metric.

Value: as it's one of the 4 more important DORA metrics, it's a MUST-HAVE for the plugin

Deliverables:

• Backend to provide the Gitlab Deployment Datafields using the out-of-the-box Apache Devlake integration
• The REST API endpoint is extended to support the ltc_* metric type and retrieve data for the frontend plugin
• The Backstage FE plugin can plot a chart for the Lead time for changes metric

Assumptions:

• The end-user/operator will setup the necessary connections to GitLab and provide the necessary transformations (if the defaults don't suffice) for determining PR -> deployment relationships.

### Tasks
- [ ] https://github.com/DevoteamNL/opendora/issues/76
- [ ] https://github.com/DevoteamNL/opendora/issues/84
- [ ] https://github.com/DevoteamNL/opendora/issues/85
- [ ] https://github.com/DevoteamNL/opendora/issues/95
- [ ] https://github.com/DevoteamNL/opendora/issues/94