refactor(minor-interchain-token-service)!: refactor custom token linking

[cjcobb23]

• no longer pass RegisterTokenMetadata nested within SendToHub. Instead, pass as its own message type.
• remove autoscaling flag

https://axelarnetwork.atlassian.net/browse/AXE-7048

[codecov]

Codecov Report

Attention: Patch coverage is 92.64706% with 5 lines in your changes missing coverage. Please review.

Project coverage is 93.68%. Comparing base (58d5bac) to head (39346c5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
contracts/interchain-token-service/src/abi.rs	90.32%	3 Missing ⚠️
...token-service/src/contract/execute/interceptors.rs	75.00%	1 Missing ⚠️
...ntracts/interchain-token-service/src/primitives.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #738      +/-   ##
==========================================
- Coverage   93.70%   93.68%   -0.02%     
==========================================
  Files         222      222              
  Lines       36319    36259      -60     
==========================================
- Hits        34033    33971      -62     
- Misses       2286     2288       +2     

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