Add method to retrieve price fields via ContributionPage.get API

Possibly just needs documenting, I don't see it.

If I use ContributionPage.get to retrieve a Contribution Page (or Event, etc), there doesn't seem to be a method in the API to directly retrieve the priceset(s) associated with that page.

In API explorer for PriceSet entity, we can select "Extends" and select a Civi entity type (eg contribution page), but not a specific entity.

There's an internal method but not an exposed API method?

CRM_Price_BAO_PriceSet::getFor('civicrm_contribution_page', $id));

# Returns contribution page data but not info about pricesets associated with
cv api ContributionPage.get
# Returns pricesets, but not clear how to filter to desired contribution page
cv api PriceSet.get sequential=1 extends="CiviContribute"

You can post-filter the priceset results, but this seems like something the API would offer.

I've implemented for my own needs here, keen to find either if there's already a way to do this (yay) or if not whether we should add to API.

https://github.com/fuzionnz/contrib.entityextra/blob/master/CRM/Entityextra/APIWrapper.php#L11