logging: Add API's to switch logging formats at runtime.

Add support for new API's log_backend_format_set and
log_format_set_all_active_backends to switch the logging format
for one backend and for all the active backends respectively.
Using format_set function pointer in log_backend_api struct as a hook
to set the log format at runtime in the backends. Add function pointer
table with helper functions to be used as a way to select log format.
Add supported log format types as macros to use with the API.

Signed-off-by: Aastha Grover <aastha.grover@intel.com>