Extract Proposal and Rfp types into a shared crate

[Tguntenaar]

Creating this issues to track since I've already published this crate.

The proxy / cache we are building to replace the indexer needed to know the structure of the RPC responses in order to satifsy the Send trait from near-api-rsy.

I kept the crate locally inside the contract to keep contract development easier. But changes made to the types will have to be published and updated in the caching repository.