CU-2p64ar7 - Update custom rpc wrapper representation

[hussein-aitlahcen]

Issue

Fix the assets.balanceOf custom RPC by moving the expected JSON representation from hex string to string (Display).

Checklist

• I have updated the cargo docs to reflect changes made by this PR (if applicable)
• I have updated the book/ to reflect changes made by this PR (if applicable)

[itsbobbyzz]

Task linked: CU-2p64ar7 Asset RPC balanceOf

[codecov-commenter]

Codecov Report

Merging #900 (a8da496) into main (d0547c7) will increase coverage by 0.06%.
The diff coverage is 5.00%.

@@            Coverage Diff             @@
##             main     #900      +/-   ##
==========================================
+ Coverage   70.76%   70.82%   +0.06%     
==========================================
  Files         215      215              
  Lines       31106    31078      -28     
==========================================
  Hits        22012    22012              
+ Misses       9094     9066      -28     

Impacted Files	Coverage Δ
frame/assets/rpc/src/lib.rs	3.57% <ø> (ø)
frame/assets/runtime-api/src/lib.rs	8.33% <0.00%> (ø)
frame/composable-support/src/rpc_helpers.rs	0.00% <0.00%> (ø)
frame/crowdloan-rewards/runtime-api/src/lib.rs	10.00% <0.00%> (ø)
runtime/dali/src/lib.rs	24.86% <0.00%> (ø)
runtime/picasso/src/lib.rs	19.20% <0.00%> (ø)
runtime/primitives/src/currency.rs	17.72% <0.00%> (-0.23%)	⬇️
frame/crowdloan-rewards/rpc/src/lib.rs	3.84% <100.00%> (ø)

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