logging: allow OOT backend to enable LOG_BACKEND_FORMAT_TIMESTAMP

The `LOG_BACKEND_FORMAT_TIMESTAMP` Kconfig currently depends on
a list of hardcoded backends.

Let's modify it to depend on an intermediary Kconfig
`LOG_BACKEND_SUPPORTS_FORMAT_TIMESTAMP` instead, which can be
selected by a OOT log backend.

Updated all exisitng supported backends to select this new
Kconfig.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>