kconfig: openthread: Add OPENTHREAD_CLI_VENDOR_EXTENSION option

This commit introduces a new option, OPENTHREAD_CLI_VENDOR_EXTENSION,
in the Zephyr OpenThread module to map the upstream option
OT_CLI_VENDOR_EXTENSION.

OPENTHREAD_CLI_VENDOR_EXTENSION option specifies the path to a CMake
file that defines and links the CLI vendor extension. By setting this
option, it enables the addition of vendor-specific commands to
the OpenThread CLI interface.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>