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`.

requests_unixsocket is now an optional dependency, only needed when connecting to redner on a unix socket

Enhance _set_value_from_template to support Redner integration

- Fix typo in `placeholderRednerField` → `placeholderRenderField`
- Validate `converterField` as string type

Requires a Odoo with [ADD] *: pypdf 3.x compatibility (https://github.com/odoo/odoo/commit/fddf53c9b6bcaea1a9ff7e041c0ccbb65a4647c8)