net: l2: Configurable headroom by drivers

[krish2718]

Is your enhancement proposal related to a problem? Please describe.
Feature is zero-copy where the network buffer is allocated only once and used by the drivers as is. But L2/drivers have requirements of adding more headers, so, they can request a fixed headroom at the registration and networking stack takes that into account while allocating.

Describe the solution you'd like
Extend https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/net/l2/ethernet/ethernet.c#L514 and rename to include a driver's headroom, need to add a new member unsigned int extra_tx_headroom in https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/net/l2/ethernet/ethernet.c#L514 (or other appropriate structure).

Describe alternatives you've considered
Driver's now re-allocate the buffer with the extra headroom and copy the net buf.

Additional context

See #83994 and #50188, all of this work towards zero-copy TX data path.