Initial implementation of SP#015 DescriptorHandle<T>. (#6028)

* Initial implementation of `ResourcePtr<T>`.

* Update docs

* Fix build error.

* Add more discussion.

* Update documentation.

* Update TOC.

* Fix.

* Fix.

* Add test case for custom `getResourceFromBindlessHandle`.

* Add namehint to generated descriptor heap param.

* Fix.

* Fix.

* format code

* Rename to `DescriptorHandle`, and add `T.Handle` alias.

* Fix compiler error.

* Fix.

* Fix build.

* Renames.

* Fix documentation.

* Documentation fix.

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Co-authored-by: slangbot <[email protected]>