feat: Rt-Api pallet-loans - Unify External- and Internal-Pricing

[mustermeiszer]

Description

We are planning on merging the currently existing Pricing::External and Pricing::Internal methods into a single pricing mechanism to unify the loans logic and allow to price all assets in both ways.

See specification here.

The problem is that from an indexing perspective this is a breaking change, so it would be desirable if the existing runtime api LoansApi would already fake this unification on the api level so that the indexer simply can use this api.

Research/based on

Communication with @hieronx and Embrio.

How will this affect the code base

Allow indexers to handle all blocks the same w.r.t loans and not have custom logic pre- and post-changes.

What are foreseen obstacles or hurdles to overcome?

• Lock in of datastructure on the api level which could be a real problem in the future.