Improve substitution management in ir.actions.report and mail.template with

value_type functionality

- 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`.