Add Benchmarking/Weights for Signed Extensions

[joepetrowski]

With the introduction of Asset Conversion for fee payment, sometimes Signed Extensions can execute "heavier" logic themselves in pre-/post-dispatch. See for example asset-conversion-tx-payment.

In this case, these pre_dispatch and post_dispatch functions should allow some weight annotation that will add the weight of their execution to the extrinsic weight and charge when appropriate.

[bkchr]

Just a note as we normally call the same logic as in pre_dispatch in validate. For this "heavier" logic signed extensions we should may move away from this. We should only do basic checks, which in the case of the asset conversion extension would be to check that the user has the asset and that there exists a pool to sell it. Maybe also some preliminarily checks on the pool liquidity.

While writing this down and thinking more about this, I'm not sure it is that easy :P Currently the BaseExtrinsicWeight is fixed and includes the weight for all signed extrinsics. This BaseExtrinsicWeight is included when calculating the weight of a call and thus, also used to calculate the fees to pay. We would need to make the BaseExtrinsicWeight "dynamic" to calculate it on the fly. It should be possible, but it will not be nice 🙈