Pysigma

What's Changed
* Added FieldPrefixMappingTransformation.
* Fix a big here a ConditionItem with multiple items would not have a p… by Res260 in https://github.com/SigmaHQ/pySigma/pull/66

New Contributors
* Res260 made their first contribution in https://github.com/SigmaHQ/pySigma/pull/66

**Full Changelog**: https://github.com/SigmaHQ/pySigma/compare/v0.8.8...v0.8.9

* Added custom Sigma rule attributes in `custom_attributes`.

* SigmaString slice with start after end of string now results in empty SigmaString instead of exception. This matches the behavior of Pytho strings.

* Fix: conversion of rule ids to UUIDs on load of validator configuration from YAML or dict.
* Fix: Added type `tool` to MITRE ATT&CK software tags.

* Improved validator docstrings for displaying in CLI.

* Improved rule error collection for checking of rules.