Refactor predefined dashboard services

What does this MR do?

We currently have two predefined dashboards: config/prometheus/common_metrics.yml and config/prometheus/cluster_metrics.yml. These dashboards are processed by Metrics::Dashboard::SystemDashboardService and Metrics::Dashboard::ClusterDashboardService respectively. These services have a lot in common. In preparation for adding another predefined dashboard in !20242 (merged), move out common code into a Metrics::Dashboard::PredefinedDashboardService that SystemDashboardService, ClusterDashboardService and future predefined dashboard services can inherit from.

Changes that this MR contains:

1. Move common code from Metrics::Dashboard::SystemDashboardService and Metrics::Dashboard::ClusterDashboardService into a new class Metrics::Dashboard::PredefinedDashboardService.

2. Rename the SYSTEM_DASHBOARD_PATH and SYSTEM_DASHBOARD_NAME constants to be DASHBOARD_PATH and DASHBOARD_NAME instead.

3. Move spec caches the unprocessed dashboard for subsequent calls into MetricsDashboardHelpers so that it can be reused in all dashboard service specs.

Issue: #35242 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team