samples: subsys: zbus: Avoid stack overflow with TLS

Enabling TLS increases stack usage by a small amount as that is where any
TLS variables are stored. When enabled, this sample ends up overflowing the
512-byte subscriber task stack and the IDLE_STACK_SIZE consumer stacks

Replace the fixed 512-byte subscriber stack allocation with
CONFIG_MAIN_STACK_SIZE so that it adopts a value controlled by the
configuration in case it needs to be adjusted further.

Increased CONFIG_IDLE_STACK_SIZE to 1024 to provide more space in each
consumer stack.

Signed-off-by: Keith Packard <keithp@keithp.com>