new (`rules-engine`): add new metadata fields to rules

[hdtrinh]

Add new metadata fields to rules

This PR implements the parsing of new metadata to enrich security rules. Currently, we would like to add these new fields:

• description: a brief description of the rule
• category: threat category (e.g. Command and Control, Persistence, etc.)
• severity: severity of the rule (e.g. low, medium, high, critical)
• mitre_tactic: MITRE ATT&CK tactic (e.g. TA0004 - Privilege Escalation - https://attack.mitre.org/tactics/TA0004/)
• mitre_technique: MITRE ATT&CK technique (e.g. T1053 - Scheduled Task/Job - https://attack.mitre.org/techniques/T1053/)

The rule syntax will be as follows:

- name: Unshadow execution start
  type: Exec
  condition: header.image ENDS_WITH "/unshadow" AND payload.argc >= 2
  category: credential_access
  severity: medium
  description: Identifies the execution of the unshadow utility which is part of John the Ripper,
    a password-cracking tool on the host machine. Malicious actors can use the utility to retrieve
    the combined contents of the '/etc/shadow' and '/etc/password' files.
    Using the combined file generated from the utility, the malicious threat actors can use them as input
    for password-cracking utilities or prepare themselves for future operations by gathering credential information of the victim.

Valid severity values are low, medium, high, and critical.

Valid category values are from the MITRE ATT&CK framework:

• command_and_control
• credential_access
• defense_evasion
• discovery
• execution
• exfiltration
• generic
• impact
• initial_access
• lateral_movement
• persistence
• privilege_escalation
• reconnaissance
• resource_development

I have

• [x] run cargo fmt;
• [x] run cargo clippy;
• [x] run cargo testand all tests pass;
• [ ] linked to the originating issue (if applicable).