refactor(misconf): move common logic to base value and simplify typed values

[nikpivkin]

Description

This PR refactors the handling of IaC values in the iac/types package. A new generic BaseValue structure is introduced, serving as a universal container for storing a value along with its metadata. It centralizes all core logic for working with values, including JSON serialization and Rego conversion.

Typed values (StringValue, BoolValue, TimeValue, etc.) remain and can override behavior, for example TimeValue.ToRego, as well as provide domain-specific methods like StringValue.StartsWith or StringValue.Contains.

Constructors are preserved for convenient and safe creation of values, while invariants such as Default, Explicit, and Unresolvable are centrally enforced through BaseValue.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[github-actions]

📊 API Changes Detected

Semver impact: major

github.com/aquasecurity/trivy/pkg/iac/types
  Incompatible changes:
  - BaseAttribute: removed
  - BoolValue.BaseAttribute: removed
  - BytesValue.BaseAttribute: removed
  - IntFromInt32: removed
  - IntValue.BaseAttribute: removed
  - MapValue.BaseAttribute: removed
  - StringValue.BaseAttribute: removed
  - TimeValue.BaseAttribute: removed
  - TimeValue.Value: changed from func() time.Time to func() RFC3339Time
  Compatible changes:
  - BaseValue: added
  - BoolValue.BaseValue: added
  - BytesValue.BaseValue: added
  - IntValue.BaseValue: added
  - MapValue.BaseValue: added
  - RFC3339Time: added
  - StringValue.BaseValue: added
  - TimeValue.BaseValue: added