Gateway should no longer build token registration XCM

[vgeddes]

Instead, I've modified the protocol so that:

1. Users can still pass raw xcm blobs
2. The gateway itself can pass a "template id" like CreateAssetXCM of raw xcm.

Then the inbound-queue on BH can either passthrough the raw xcm or build the create asset XCM.

[yrong]

@vgeddes @claravanstaden Since we've already merged vgeddes/polkadot-sdk#13, we may also need to merge this PR to V2 to pass the smoke tests.

I've resolved the conflicts in recent commits, Let me know if we need any further tweaks!

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (v2@7b0d54e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
contracts/src/Gateway.sol	0.00%	1 Missing ⚠️
contracts/src/v2/Calls.sol	50.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             v2    #1381   +/-   ##
=====================================
  Coverage      ?   73.16%           
=====================================
  Files         ?       23           
  Lines         ?      898           
  Branches      ?      135           
=====================================
  Hits          ?      657           
  Misses        ?      221           
  Partials      ?       20           

Flag	Coverage Δ
solidity	73.16% <33.33%> (?)

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