Create an IBT entity to get more IBT Rate precision

[jeanchambras]

• Create a new IBT entity that will udpate the IBT rate and referenced in all PT
• This IBT entity is updated upon each IBT transfer BUT we don't index / store each transfer because this would be too large and costly to index all these entitites. The only thing we modify is the ibtRate
• Some optimisations are done to avoid some eth_call with assets

We will then in the app fetch the IBTRate from the IBT entity as this value will be the most updated

[matstyler]

why do we need underlying unit to calculate ibt rate?

[jeanchambras]

This is the definition of the IBT Rate it is the exchange rate between the IBT and the underlying given in underlying decimals. Here we give the raw result and also a precomputation of the rate for readability, and for different use cases where we don't need a BigInt. For some computation in the app the IBT Rate is used with a BigInt computation and is thus faster to already have a BigInt and reduce to a float at the end of the computation