CookAdditionalFiles - WriteSharedFile function that allows multiple packages to generate the same file

Dependency on [Link Removed], which specifies that CookAdditionalFiles should take a context object.

Keep the existing WriteAdditionalFile function that is an argument to CookAdditionalFilesOverride on the context object, but rename it to WriteUniqueFile. Files created by WriteUniqueFile are chunked in with the package that created them no matter what their path is. Multiple objects writing to the same filename is an error.

Add another function to the context object: WriteSharedFile. WriteSharedFile receives a UE::DerivedData::FBuildDefinition rather than an array of bytes. The function name in the FBuildDefinition has to match a unique function that modules can register during StartupModule. It takes the FBuildDefinition as input and is not allowed to read any data other than the files,constants,etc provided in the BuildDefinition). Providing multiple non-equal BuildDefinitions that write to the same filename is an error. WriteSharedFile files are chunked in with all the packages that write them.