Get token prices from our API instead of CoinGecko

CoinGecko is planning on retiring their `/simple/token_price` endpoint
soon. This endpoint is critical for us, as we use to fetch prices for
tokens. This commit removes all code related to CoinGecko in
TokenRatesController so that it hits an internal API instead.

This is set up in such a way that we can upgrade to newer versions of
the API (or even other services) in the future without breaking backward
compatibility. However, at the moment it does introduce breaking
changes to TokenRatesController.

packages/assets-controllers/CHANGELOG.md

@@ -5,6 +5,27 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- Add `codefiTokenPricesServiceV2`
+  - This object can be used for the aforementioned `tokenPricesService` argument for TokenRatesController. It uses an internal API to fetch prices for tokens instead of CoinGecko.
+
+### Changed
+- **BREAKING:** TokenRatesController now takes a required argument `tokenPricesService` ([#3600](https://github.com/MetaMask/core/pull/3600))
+  - This object is responsible for fetching the prices for tokens held by this controller.
+- **BREAKING:** Update signature of `TokenRatesController.updateExchangeRatesByChainId` ([#3600](https://github.com/MetaMask/core/pull/3600))
+  - Rename `tokenAddresses` argument to `tokenContractAddresses`
+  - Change the type of `tokenContractAddresses` from `string[]` to `Hex[]`
+- **BREAKING:** Change signature of `TokenRatesController.fetchAndMapExchangeRates` ([#3600](https://github.com/MetaMask/core/pull/3600))
+  - This method now takes an object with shape `{ tokenContractAddresses: Hex[]; chainId: Hex; nativeCurrency: string; }` rather than positional arguments
+
+### Removed
+- **BREAKING:** Remove `fetchExchangeRate` method from TokenRatesController ([#3600](https://github.com/MetaMask/core/pull/3600))
+  - This method was only ever intended to be used internally and should not be accessed directly.
+- **BREAKING:** Remove `getChainSlug` method from TokenRatesController ([#3600](https://github.com/MetaMask/core/pull/3600))
+  - This method was only ever intended to be used internally and should not be accessed directly.
+  - This method was previously used in conjunction with the CoinGecko API, which has been replaced with an internal API.
+- **BREAKING:** Remove `CoinGeckoResponse` and `CoinGeckoPlatform` types ([#3600](https://github.com/MetaMask/core/pull/3600))
+  - There is no equivalent to these.
 
 ## [20.0.0]
 ### Added