Allow setting Unlimited for BuyExecution in XCM Transactor

[Agusrodri]

What does it do?

Modifies the overall_weight field inside TransactWeights struct of pallet-xcm-transactor to allow setting Unlimited as the total weight to buy in the destination chain.

It also adds the possibility of using Unlimited through the XCM Transactor precompile V3. To do this, a uint64::MAX should be passed through the refTime field of the overall_weight parameter in the desired function of the precompile's smart contract.

[librelois]

Some naming conventions are confusing

[github-actions]

Coverage Report

@@                         Coverage Diff                         @@
##           master   agustin-unlimited-buy-execution      +/-   ##
===================================================================
- Coverage   81.90%                            81.88%   -0.02%     
  Files         286                               286              
+ Lines       95261                             95320      +59     
===================================================================
+ Hits        78018                             78049      +31     
+ Misses      17243                             17271      +28     

Files Changed	Coverage
/pallets/xcm-transactor/src/lib.rs	88.48% (-0.52%)	🔽
/precompiles/xcm-transactor/src/functions.rs	96.80% (-0.71%)	🔽
/runtime/common/src/migrations.rs	80.11% (-5.32%)	🔽
/runtime/moonbase/src/lib.rs	50.13% (-0.25%)	🔽
/runtime/moonbeam/src/lib.rs	69.05% (-0.25%)	🔽
/runtime/moonriver/src/lib.rs	68.97% (-0.25%)	🔽

Coverage generated Mon Dec 11 13:30:05 UTC 2023