shell: add Kconfig option for configurable autoflush behavior

Introduced a new Kconfig option `SHELL_PRINTF_AUTOFLUSH` to allow
configuring the autoflush behavior of shell printing functions.

Updated `Z_SHELL_FPRINTF_DEFINE` to use the
`CONFIG_SHELL_PRINTF_AUTOFLUSH` setting instead of hardcoding
the autoflush behavior to `true`.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>