feat: add castai_workload_custom_metrics_data_source resource

[damiankaminski]

Summary

• Add castai_workload_custom_metrics_data_source resource to the module, allowing users to configure Prometheus custom metrics data sources via the workload_custom_metrics_data_sources variable
• Supports presets (e.g. jvm) and manually defined PromQL metrics
• Bump minimum provider version to >= 8.26.0

Related provider: castai/terraform-provider-castai#669

Test plan

• terraform validate passes
• Verify docs are correctly generated

[dvoros-castai]

Nit: there's no depends_on = [helm_release.castai_workload_autoscaler] here as opposed to castai/terraform-castai-eks-cluster#173, I assume that's not intentional.

Context: trying to keep these modules in sync with the goal of extracting these pieces to a shared sub-module some day. Small diffs like this will make that harder to do.

[damiankaminski]

Nit: there's no depends_on = [helm_release.castai_workload_autoscaler] here as opposed to castai/terraform-castai-eks-cluster#173, I assume that's not intentional.

Context: trying to keep these modules in sync with the goal of extracting these pieces to a shared sub-module some day. Small diffs like this will make that harder to do.

Good catch! Added here and in the other PR (it was missing in 2 out of 3 PRs)