Enhance substitution handling in ir.actions.report with value_type support

• Refactor converter building functionality for improved maintainability.

• Added substitution_mixin.py with the new SubstitutionMixin abstract model.

• Updated ir.actions.report to inherit from SubstitutionMixin and implement _get_template to return redner_tmpl_id.

• Updated mail.template to inherit from SubstitutionMixin, replacing its simpler action_get_substitutions with the mixin's implementation, and added _get_template.