Bluetooth: Mesh: add Kconfig options for BLOB chunk size

The MBT server, as the MBT client currently sets the maximum chunk size
according to maximum supported segments in the accesss layer. This might
be suboptimal for some use cases.
The added Kconfig options give customers the option to fine tune it
themselves.

Future work will include addition of an API for the customer to modify
it also during runtime.

Signed-off-by: Kyra Lengfeld <kyra.lengfeld@nordicsemi.no>