bluetooth: shell: add `bt_shell_private.c` and `bt_shell_private.h`

Introduced `bt_shell_private.c` and `bt_shell_private.h` to provide
common functions for the Bluetooth `shell_wall_print`.
These functions are equivalent to `shell_fprintf`, `shell_info`,
`shell_print`, `shell_warn`, `shell_error` and `shell_hexdump`
but without requiring the `sh` parameter.

The cost of the newly added `bt_shell_fprintf_info` ... `_error` functions
will be negligible when there are many individual calls that need to pass
both the `sh` and `color` parameters each time.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>