[Feature][Plugin] Add Grafana IRM Integration

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/incubator-devlake/issues?q=is%3Aissue) and found no similar feature requirement.

### Use case

Teams using Grafana Cloud IRM should be able to ingest its data into Apache DevLake and analyze it alongside data from other DevOps tools.

For example, incidents managed in Grafana IRM could be correlated with deployment data and used to calculate DORA metrics such as:
- Change Failure Rate
- Failed Deployment Recovery Time / MTTR

### Description

I propose adding a `grafana_irm` data source plugin to support Grafana Cloud IRM.

Grafana IRM provides multiple capabilities, including Incident and OnCall, with APIs for programmatic access:

- [Grafana IRM Incident API](https://grafana.com/docs/grafana-cloud/alerting-and-irm/irm/reference/incident-api/)
- [Grafana IRM OnCall API](https://grafana.com/docs/grafana-cloud/alerting-and-irm/irm/reference/oncall-api/)

For example, Grafana IRM Incident data could be mapped to DevLake's domain models as follows:

- Grafana IRM Incident → DevLake Issue (`type = INCIDENT`)
- Incident status → Issue status / original status
- Incident severity → Issue severity / priority
- Incident declared time → Issue created date
- Incident resolved time → Issue resolution date
- Incident roles/members → Issue assignees, where applicable

This could allow Grafana IRM incidents to be correlated with deployment data through DevLake's existing DORA incident-to-deployment correlation.

Before proceeding with the implementation, I would like to confirm whether this overall direction is appropriate, particularly regarding:

- which Grafana IRM APIs and entities should be supported
- the scope of data to be collected
- the synchronization strategy

### Related issues

No related issue found.

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Start with the linked Grafana IRM Incident API and OnCall API documentation, then compare their available entities with DevLake's existing data-source plugin patterns. The issue is ready to move forward when maintainers agree on the supported APIs and entities, collected scope, and synchronization strategy.

Written by the indexing model from the issue text.