Refactor Orderbook Offchain RPC

[Gauthamastro]

Describe your changes

The recent substrate upgrade broke our Orderbook offchain RPCs. This PR fixes that.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• I removed all Clippy and Formatting Warnings.
• I added required Copyrights.

[codecov]

Codecov Report

Attention: 114 lines in your changes are missing coverage. Please review.

Comparison is base (d803ec1) 37.52% compared to head (9eddb17) 36.92%.

Files	Patch %	Lines
pallets/ocex/rpc/src/lib.rs	0.00%	104 Missing ⚠️
pallets/ocex/src/rpc.rs	0.00%	6 Missing ⚠️
runtimes/mainnet/src/lib.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           Develop     #886      +/-   ##
===========================================
- Coverage    37.52%   36.92%   -0.60%     
===========================================
  Files           62       62              
  Lines         3750     3810      +60     
===========================================
  Hits          1407     1407              
- Misses        2343     2403      +60     

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[zktony]

Lgtm