logging: v2: add kconfig to always add indexes to msg packages

This adds a new CONFIG_LOG2_MSG_PKG_ALWAYS_ADD_RO_STRING_IDXS
kconfig. If enabled, the log message packages will always have
the indexes of read-only string arguments appended to the package.
This will be selected only by those backends requiring it.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>