Enable User-Defined Header Sorting

• Add an option to SfgComposer for the user to define a partial order according to which #include statements should be sorted.
• Make sure clang-format does not undo that sorting:
  • Clang-format has a --sort-includes flag. Add an option to ClangFormatOptions to configure includes sorting.
  • If a user-defined order is provided, always disable automatic include sorting by clang-format.