[New Integration] Add ReConfirm integration

[m11tch]

Suggested label: Enhancement

Proposed commit message

[New Integration] Add ReConfirm integration

Adds the ReConfirm integration to collect attack-surface and vulnerability
scan results via webhook and normalize them to ECS.

WHAT:

• Adds a new vulnerability data stream backed by an http_endpoint input;
  ReConfirm POSTs one JSON document per completed scan.
• Reshapes ReConfirm's dynamically keyed sections, including assets by IP,
  subdomains by hostname, vulnerability findings, associated domains, and
  credential leaks, into bounded nested arrays instead of unbounded dynamic
  mappings.
• Maps vulnerability findings onto ECS vulnerability.* fields, including
  severity, CVE ID, and CVSS score, alongside ReConfirm-specific fields under
  reconfirm.*.
• Adds redact_leaked_passwords, enabled by default, to replace leaked
  plaintext credential values with REDACTED in both
  reconfirm.cred_leaks.password and event.original.

WHY:

• Lets security teams track external attack-surface findings, exposed
  services, lookalike domains, leaked credentials, and email security posture
  in Elasticsearch alongside their other telemetry.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

N/A: no dashboards are included in this version.

Author's Checklist

• Update changelog.yml's PR link from the pull/1 placeholder to this PR's actual number once opened.

How to test this PR locally

cd packages/reconfirm
elastic-package build
elastic-package format --fail-fast
elastic-package lint
elastic-package check
elastic-package test pipeline
elastic-package test system --generate
elastic-package test system

All commands passed locally.

Related issues

Screenshots

N/A: no dashboards or visual UI assets are included in this version.

[elastic-vault-github-plugin-prod]

Reviewers

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NOTE: https://github.com/elastic/integrations/blob/main/.buildkite/pull-requests.json contains all those details.