Add junit output format for check results

[dmnk]

Creating an JUnit xml result of the check would make it easier to integrate sigma-cli into a CI/CD Pipeline and have a nice representation of the results / work on them.

Possible integrations (for possibly slightly different output formats):

• https://medium.com/@ctrf-io/github-actions-publish-and-view-test-results-report-23bf4f3872ff (they work on CTRF - Common Test Report JSON Files)
• https://docs.gitlab.com/ci/testing/unit_test_reports/ (working on JUnit XML result format ) - but the more I research the more I like the idea to (in the mid-term) integrate code quality results (utilizing the code climate report format)

use case:

• ensure a specific minimum level of quality of sigma rules in a repository
• have an overview of possible regressions when changing code

[omobolajiadeyan]

I would like to implement this. After reviewing sigma/cli/check.py, I propose keeping the terminal output as the default and adding:

• --format text|junit (default: text)
• --output PATH for machine-readable output; JUnit writes to stdout when omitted
• one <testsuite name="sigma-check">
• a <testcase> per parsing error, condition error, and validation issue
• validation severity and affected rule paths in the failure message/body
• the existing --fail-on-error and --fail-on-issues behavior remains the source of the process exit code

Implementation would use xml.etree.ElementTree from the standard library and add tests for valid rules, parsing errors, validation issues, escaping, file output, and unchanged exit behavior.

If --format/--output matches the intended CLI shape, I can submit the focused implementation and tests.