Add support for audit_attributes

[nazamoresco]

In the current project I'm working on, we introduced a new column to the audit class but struggled to find an elegant way to set it dynamically. A feature similar to audit_comment but more flexible would provide a more convenient solution.

This PR introduces the audit_attributes feature to the gem, offering a flexible approach to include additional custom attributes during the auditing process. The following changes have been made:

• Added audit_attributes to track custom attributes during auditing.
• Implemented validation to ensure audit_attributes is a hash containing only valid keys from the audit class.
• Enhanced the audit creation, update, and destroy methods to include audit_attributes in audit records.
• Updated specs to test the inclusion of audit_attributes in audit records and validate its correctness.
• Modified the schema to include a new custom_attribute column in the audits table.
• Ran the linter to ensure code quality.