Refactor the whole sale price computation logic in `product` & `sale` to improve code quality, maintainability, ...

### Drop/reduce as much as possible context-based logic

* Drop magic `price` and `pricelist_id` fields on `product.product`/`product.template`
  * Keep one unique `_get_contextual_price()` to provide the old logic where it's still needed/difficult to remove atm.
* Do not rely on context fallbacks on the lower level of price computation (aka `price_compute` method)
  It is way too dangerous to have such magic fallbacks so deeply in the logic, inducing potentially unexpected/wrong results

### Clean `product.pricelist` API

* Clear parameters and fallbacks
  * Do not provide unused parameter(s) anymore (`partner`)
  * Stop providing products & quantities as a list of tuples, it was never used with different quantities in the existing code anyway
* Removed unused methods
* Split methods logic to ease and allow clean overrides and customizations
* Convert the old hardcoded SQL query to a clear domain, using ORM querying and formatting logic instead
* Improved documentation (docstrings, ...)

### Side cleanups in related/impacted modules

* Update to use the new clean pricelist API
* Code cleanup in sale prices computation (`sale_*`, `website_sale`, ...)
* Some onchanges -> computes conversion in `sale_*` scope
* Cache applied pricelist rule on SOL (avoid recomputing the pricelist rule in the price AND in the discount computation)

Task ID - 2688106
Community PR: https://github.com/odoo/odoo/pull/79605
Enterprise PR: https://github.com/odoo/enterprise/pull/22211

<!-- Keep both authorship for the merge commit (author=robodoo) -->

Co-authored-by: Victor Feyens <vfe@odoo.com>
Co-authored-by: Yannick Tivisse <yti@odoo.com>

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