Added support for additional validation rules in custom detector

[kashifkhan0771]

Description:

This PR adds support for validation rules in custom detectors to reduce false positives.

Problem:
Go's RE2 regex engine lacks support for lookahead assertions (e.g., (?=.*[0-9])), making it impossible to validate that detected secrets contain specific character types like digits, uppercase letters, or special characters. This limitation often leads to false positives when detecting API keys, passwords, and tokens that should follow specific rules.

Solution:
Added a new validations field to the Custom detector configuration that supports the following validation types:

• contains_digit: Requires at least one numeric digit (0-9)
• contains_lowercase: Requires at least one lowercase letter (a-z)
• contains_uppercase: Requires at least one uppercase letter (A-Z)
• contains_special_char: Requires at least one special character

Example Usage:

detectors:
  - name: API Key Detector
    keywords:
      - api_key
    regex:
      key: ([A-Za-z0-9]{32,})
    validations:
      key:
        contains_digit: true
        contains_uppercase: true
        contains_special_char: true

Checklist:

• Tests passing (make test-community)?
• Lint passing (make lint this requires golangci-lint)?

[nabeelalam]

LGTM!

[amanfcp]

LGTM