[XCM-Transactor] Make fee_payer field optional in transact_through_sovereign

[Agusrodri]

What does it do?

Changes fee_payer field type from T::AccountId to Option<T::AccountId>. This allows root origin to avoid specifying a fee payer when executing transact_through_sovereign function of pallet-xcm-transactor.

If fee_payer is set to None, the XCM execution fees will be paid by the sovereign account on the destination chain.

Note

It's important to properly fund the sovereign account on destination with the corresponding amount that will be used to pay for execution fees, otherwise the call will fail or also there could be some funds mismatch.

[github-actions]

Coverage Report

@@                     Coverage Diff                     @@
##           master   agustin-fee-payer-field      +/-   ##
===========================================================
- Coverage   72.51%                    72.50%   -0.01%     
  Files         229                       229              
+ Lines       70565                     70651      +86     
===========================================================
+ Hits        51164                     51219      +55     
+ Misses      19401                     19432      +31     

Files Changed	Coverage
/pallets/moonbeam-lazy-migrations/src/lib.rs	58.33% (-29.17%)	🔽

Coverage generated Wed Mar 27 13:01:01 UTC 2024

[RomarQ]

Lets only merge it after #2700 and after changing the target branch to master.

[RomarQ]

Lets have this change rebased with master, you can create a new PR if you want to avoid a force push.

[Agusrodri]

Lets have this change rebased with master, you can create a new PR if you want to avoid a force push.

Already rebased without force pushes :)