feat(uptimerobot): migrate provider to UptimeRobot API v3

[davidgrldo]

Fixes #637

What

Migrates the UptimeRobot provider from the legacy v2 API to the v3 REST API.

Changes

• Auth: Authorization: Bearer <apiKey> header on every request (api_key no longer sent in form bodies)
• REST endpoints: GET/POST/PATCH/DELETE /v3/monitors and /v3/psps with JSON bodies (replaces v2 verb-style POST endpoints)
• Pagination: cursor-based via nextLink (replaces offset/limit)
• Error handling: HTTP status codes (v2 stat/error envelope is gone)
• Rate limiting: bounded retry honoring Retry-After, capped at 3 retries (also fixes the previously unbounded recursive retries in Add/Update)
• Status pages: attach/detach via PATCH /psps/{id} with the full monitorIds array; Remove now detaches a monitor from its status pages before deleting it
• CRD unchanged: all existing uptimeRobotConfig fields keep their user-facing format (alertContacts id_threshold_recurrence-, maintenanceWindows, customHTTPStatuses) and are mapped to v3 structured fields internally
• Tests: 16 new httptest-based unit tests for the v3 client (auth header, JSON bodies, pagination, keyword/alertContacts mapping, PSP attach/detach, 429 retry); live-API integration tests updated to v3 semantics, still self-skip without config
• Docs: docs/uptimerobot-configuration.md, CONTRIBUTING.md, and example configs updated to the v3 apiURL

Verification

• go build ./... ✅
• go vet ./pkg/http/... ./pkg/monitors/uptimerobot/... ✅
• go test ./pkg/... (all packages, offline) ✅
• gofmt clean ✅

Notes for reviewers

• v3 requires timeout on monitor creation; a DefaultTimeout = 30 constant is used.
• The v2 custom_http_statuses format (200:0_401:1) does not map 1:1 to v3 successHttpResponseCodes — codes flagged :1 become success codes; noted in a code comment.
• No CRD/schema changes — new v3-only features (tags, custom headers, request body, etc. from [ENHANCE] UptimeRobot all monitors features #603) can follow in a separate PR once this migration lands.