fix(datadog): allow specifying Datadog site while keeping default US1

[BillyWeans]

Description

Summary

The Datadog alerter currently hardcodes https://api.datadoghq.com/api/v1/events as the events endpoint. Datadog operates multiple regional sites (US1, US3, US5, EU1, AP1, etc.), each with a distinct API hostname. Users on non-default Datadog sites cannot use the built-in Datadog alerter without patching the source.
This PR adds an optional datadog_api_url rule parameter that allows the endpoint to be overridden. The default value is https://api.datadoghq.com/api/v1/events, preserving full backwards compatibility.

Changes

• elastalert/alerters/datadog.py — read datadog_api_url from rule config in init, use self.dd_api_url in alert() instead of hardcoded string
• elastalert/schema.yaml — add datadog_api_url: {type: string} to Datadog section
• tests/alerters/datadog_test.py — add test_datadog_alerter_custom_api_url and test_datadog_alerter_default_api_url
• docs/source/alerts.rst — document new optional parameter
• CHANGELOG.md — add entry for new feature

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

[jertel]

Thanks for submitting the PR. Please update this line to use the pattern shown in the other release sections in this document:

• - - @author

[BillyWeans]

Corrected. Apologies!