Multiple-apply schemas don't have a concise way to access all occurrences on a prim

[robpieke]

(creating placeholder issue for work I'm already doing and will submit PR for)

Description of Issue

Single-apply schemas provide a Get(prim) method and multiple-apply schemas provide a Get(prim, name) method, but if you want to iterate over all the applications on a prim and don't know the list of names, there's no concise solution.

UsdCollectionAPI has a bespoke GetAllCollections(prim) method, so I propose a generic GetAll(prim) method provided as part of the codegenTemplates

[jilliene]

Filed as internal issue #USD-7220