Pysigma

* Generalized SigmaValidator.validate_rule_collection method into validate_rules
* Added intrusion sets and malwares to ATT&CK validation

What's Changed
* Rule validation framework and validators.
* Fix crash at calling `startswith`/`endswith` on empty strings by alexott in https://github.com/SigmaHQ/pySigma/pull/62

New Contributors
* alexott made their first contribution in https://github.com/SigmaHQ/pySigma/pull/62

**Full Changelog**: https://github.com/SigmaHQ/pySigma/compare/v0.8.1...v0.8.2

Fixes:

* Field name condition tracking only adds processing item to tracking set if transformation does change fields.
* Field name condition matching distinguished for field names and detection items.
* FieldNameProcessingItemAppliedCondition now checks on field name and detection item processing set depends if a field name or detection item is checked.

* Added field name processing conditions that are also applied to the Sigma rule field name list.
* Breaking change: IncludeFieldCondition and ExcludeFieldCondition are now field name conditions and have to be moved from the `detection_item_conditions` to the new `field_name_conditions` list of a processing item.
* Tracking of applied processing items on field names. Added `FieldNameProcessingItemAppliedCondition` field name condition that checks if a processing item was applied to a field name.

* Various bugfixes
* Fail on deprecated pipe condition syntax

* Added field name mapping tracking to ProcessingPipeline.field_mappings
* Added mapping of fields list in Sigma rules