[extrinsic] fix extrinsic codec for AssetHubExtensions

[clangenb]

I am implementing in a naive way our custom signed extensions, and while doing that I noticed that the asset hub signed extensions didn't work. This PR fixes that. I am doing this in the quick way because I know that you are implementing #359, which will probably change this part of the code anyhow.

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (6929fc8) 46.98% compared to head (2a3d552) 46.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
- Coverage   46.98%   46.91%   -0.07%     
==========================================
  Files         164      164              
  Lines        8914     8926      +12     
==========================================
  Hits         4188     4188              
- Misses       4726     4738      +12     

Flag	Coverage Δ
polkadart	16.85% <0.00%> (-0.17%)	⬇️
polkadart_cli	12.05% <ø> (ø)
polkadart_keyring	77.24% <ø> (ø)
polkadart_scale_codec	54.93% <ø> (ø)
sr25519	85.91% <ø> (ø)
ss58	96.87% <ø> (ø)
substrate_bip39	63.35% <ø> (ø)
substrate_metadata	87.61% <ø> (ø)

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Files	Coverage Δ
...ges/polkadart/lib/extrinsic/extrinsic_payload.dart	0.00% <0.00%> (ø)
...kages/polkadart/lib/extrinsic/signing_payload.dart	0.00% <0.00%> (ø)