Appendices should be optional

Right now, the specification does not provide much guidance whether appendices should be loaded.

The behavior of CMake 4.0 is to try to load appendices that provide requested components, or to try to load all appendices when not given specific component requests.

It might be useful for packages to have control over this latter behavior, e.g. an `optional` attribute that specifies that an appendix should be ignored unless it provides a requested component.

No files or tests are named. Start by reviewing the specification's guidance on appendix loading and compare it with the CMake 4.0 behavior described in the issue. Define the semantics of an optional appendix, including when it is ignored and how requested components affect loading.

Written by the indexing model from the issue text.