Loans: Support for benchmarking prices

[lemunozm]

Description

• Added a MaxCollectionSize associated type that is only used under runtime-benchmarks.

By now, I think always using the maximum size of a price collection in benchmarks is ok. In comparison with the work done in portfolio_valuation over each ActiveLoan, deserializing once a small or a big vector of prices should not affect too much the final value.

If in the future, we need to struggle more with the benchmarks, we can add this as a variable parameter.

Associated to #1279

[hieronx]

I would have expected that we needed to enforce the MaxCollectionSize somewhere, e.g. when creating a new loan with a specific price ID. Is there already somewhere implicitly checked?

[lemunozm]

The price collection is only increased when a new price is registered using DataCollector::register_id() once a loan becomes active. Inside the register_id() is the check I think you're looking for.

[lemunozm]

Rebased