NVIDIA: Add prim flag HasAbstractSpecifier to filter prim with class specifier

[roggiezhang-nv]

Description of Change(s)

There's no way to identify a concretely define subgraph using traversal predicates because "IsAbstract" will match all ancestors.
This PR adds a HasAbstractSpecifier similar to HasDefiningSpecifier as a prim flag.

Example

class "abstract_root" {
    def "concrete_subgraph" {
         def "concrete_child" {
             def "concrete_grandchild" {
             }
         }
    }
}

/abstract_root/conrete_subgraph is defined but abstract because of its abstract ancestor. A HasAbstractSpecifier prim flag would allow the specification to enabling traversal of the decendants of concrete_subgraph using the predicate HasDefiningSpecifier && !HasAbstractSpecifier.

See discussion in https://forum.aousd.org/t/pointinstancer-prototype-scope-best-practices/2662.

Link to proposal (if applicable)

Fixes Issue(s)

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[roggiezhang-nv]

@nvmkuruc for vis.

[jesschimein]

Filed as internal issue #USD-11482

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[nvmkuruc]

I think it was correct to leave or classes here.