shell: uart: multi-instance implementation is broken

[ycsin]

Is your enhancement proposal related to a problem? Please describe.
Previously, it was possible to create another shell UART instance by:

SHELL_UART_DEFINE(shell_transport_uart1,
		  CONFIG_SHELL_BACKEND_SERIAL_TX_RING_BUFFER_SIZE,
		  CONFIG_SHELL_BACKEND_SERIAL_RX_RING_BUFFER_SIZE);
SHELL_DEFINE(shell_uart1, "uart1:~$ ",
	     &shell_transport_uart1,
	     CONFIG_SHELL_BACKEND_SERIAL_LOG_MESSAGE_QUEUE_SIZE,
	     CONFIG_SHELL_BACKEND_SERIAL_LOG_MESSAGE_QUEUE_TIMEOUT,
	     SHELL_FLAG_OLF_CRLF);

This isn't possible anymore after #63967, as it removed the SHELL_UART_DEFINE and other macros required to create another instance of it.

Describe the solution you'd like

Exposing some of the structures (i.e. struct shell_uart) and rework the implementation in #63967 to recreate macros that create the buffers necessary for a shell UART instance in the public shell_uart.h header, so that we can create another shell UART instance with ease again.

Describe alternatives you've considered
Implement something similar to the UART logging backend in #64917

[ycsin]

Adding Regression tag as the macros were removed from a public header, any downstream application that depended on that would break