Pysigma

Silent plugin installation.

Added test pipelines and extended test backend.

Updated plugin directory to stable version 1.

What's Changed
Breaking Changes
* Added plugin system for backends, pipelines and validators. Requires additional metadata from plugin to enable plugin system to discover it.
* Moved builtin validators to package `sigma.validators.core`.
* Reintroduced Windows logsource mapping change that introduces lists to `windows_logsource_mapping` for Sigma log sources that map to multiple Windows log sources. Use `generate_windows_logsource_items()` function from `sigma.pipelines.common` to generate processing pipelines from `windows_logsource_mapping`. It handles multivalue cases correctly.

Further Changes
* Added support for value lists in `AddConditionTransformation` processing pipeline operation.
* Update mitre attack by andurin in https://github.com/SigmaHQ/pySigma/pull/96
* docs: clarification on cidr value modifier IPv4 limitation by phantinuss in https://github.com/SigmaHQ/pySigma/pull/100

New Contributors
* phantinuss made their first contribution in https://github.com/SigmaHQ/pySigma/pull/100

**Full Changelog**: https://github.com/SigmaHQ/pySigma/compare/v0.8.12...v0.9.0

Added option to keep regular expression escpae character itself unescaped in regular expression escpae() method as well as in the text query base class.

Fix: reverted change that introduced breaking change, lists in windows_logsource_mapping.