[clang] [clang-format]: Add `Custom` to `ShortFunctionStyle`; add new AllowShortFunctionsOnASingleLineOptions for granular setup (PR #134337)

[Owen Pan via cfe-commits]

owenca wrote:

> When I changed the `AlignConsecutiveAssignments` option, I was told to reuse the same option name. Now the program allows the user to specify the option as either a string or an associative array.

Reusing the option name is IMO better than adding `SFS_Custom`. @irymarchyk is it doable?

https://github.com/llvm/llvm-project/pull/134337