Move exchange_rates to be a free function, and remove unneeded APIs.
GraphQL APY code is reworked in #17701.
 
This PR needs to be merged first, before #17701. Note that the diff
messes things up. The code was not changed, except for `self -> state`.

## Description 

Describe the changes or additions included in this PR.

## Test plan 

How did you test the new or updated feature?

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## Release notes

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- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK: