feat: Add prometheus.createSilence and prometheus.expireSilence

[samuel-asleep]

Implements two new Prometheus Alertmanager action components for managing silences in workflows.

Backend (pkg/integrations/prometheus/)

• client.go: Added CreateSilence(SilencePayload) (string, error) (POST /api/v2/silences) and ExpireSilence(silenceID string) error (DELETE /api/v2/silence/{silenceID}). Refactored execRequest to delegate to new execRequestWithBody which sets Content-Type: application/json for requests with a body. Added Matcher, SilencePayload, and silenceResponse structs aligned with the Alertmanager OpenAPI spec (isRegex required, isEqual optional with default true).
• create_silence.go: Validates matchers (non-empty, name+value required), parses duration via time.ParseDuration, computes startsAt = now / endsAt = now + duration in RFC3339, emits prometheus.silence payload, stores CreateSilenceNodeMetadata{SilenceID}.
• expire_silence.go: Accepts silenceID (supports expressions for workflow chaining), calls ExpireSilence, emits prometheus.silence.expired payload, stores ExpireSilenceNodeMetadata{SilenceID}.
• prometheus.go: Registered both components in Components().
• example.go + JSON files: Added embedded example outputs for both components.

Frontend (web_src/src/pages/workflowv2/mappers/prometheus/)

• create_silence.ts: Node metadata shows silence ID (bell-off) and matcher count; execution details show full silence info.
• expire_silence.ts: Node metadata shows silence ID (bell); execution details show ID and status.
• index.ts: Registered both mappers and added eventStateRegistry entries ("created", "expired").
• types.ts: Added CreateSilenceConfiguration, ExpireSilenceConfiguration, PrometheusSilencePayload, and PrometheusMatcher interfaces.

Demo Video

2026-02-22.12-59-59.mp4

partially implements #3083 will create a second pr to complete it