Feat: Add new oracle key

[mustermeiszer]

Description

The DYF will have shares of other funds as its assets. Those fund holdings are valued by an oracle, but have not a valid registered ISIN we could use. Hence, this fallback key will allow to specifically value a given object in the pallet-loans.

Changes and Descriptions

• Adds a new variant in enum OracleKey
• Fixes all variants by using #[codec(index = ...)

Checklist:

• I have added Rust doc comments to structs, enums, traits and functions
• I have made corresponding changes to the documentation
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.84%. Comparing base (ba41880) to head (68d40ff).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1841      +/-   ##
==========================================
- Coverage   46.87%   46.84%   -0.03%     
==========================================
  Files         165      165              
  Lines       12950    12950              
==========================================
- Hits         6070     6067       -3     
- Misses       6880     6883       +3     

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[lemunozm]

LGTM!