[P2] event-exporter: add field selector support for events

[erain]

Problem

Filtering is limited to label selectors. There is no way to reduce event volume by field selectors (e.g., type=Warning, involvedObject.kind=Pod), which is a common operational need in large clusters.

Proposed optimization

Add configuration for ListOptions.FieldSelector in the event watcher to allow server-side filtering. This reduces API load and export volume for users who only need a subset of events.

Notes / references

• Field selectors: https://kubernetes.io/docs/concepts/overview/working-with-objects/field-selectors/
• ListOptions: https://pkg.go.dev/k8s.io/apimachinery/pkg/apis/meta/v1#ListOptions

Acceptance criteria

• New flag or config option to set field selector for events.
• Field selector applied to both list and watch requests.
• Documentation updated with examples (e.g., type=Warning).