Optimize the max PoV in relation to block fullness

[ashutoshvarma]

After the introduction of https://github.com/paritytech/substrate/pull/13958 during the v1.1.0 uplift, now the PoV is taken into account by transcation-payment pallet for adjusting fee and tx priority.

Currently we use max pov value from polkadot_primitives::MAX_POV_SIZE which caters more to the relay chain than to our block fullness.

This would need good understanding of existing tokenomics