feat: add Solana to the polled exchange rates (#4914)

## Explanation

As we aim to support more chains in the MetaMask extension we will need
the exchange rates of the native tokens of these chains to properly
value a user's account. Let's add Solana to the cryptocurrencies that we
need to keep track of price.

## References

Closes
[SOL-36](https://consensyssoftware.atlassian.net/jira/software/projects/SOL/boards/718/backlog?selectedIssue=SOL-36)

## Changelog

### `@metamask/assets-controller`

- feat: add Solana to the polled exchange rates

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've highlighted breaking changes using the "BREAKING" category
above as appropriate
- [x] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes


[SOL-36]:
https://consensyssoftware.atlassian.net/browse/SOL-36?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

packages/assets-controllers/src/RatesController/RatesController.test.ts

@@ -100,8 +100,14 @@ describe('RatesController', () => {
       const { fiatCurrency, rates, cryptocurrencies } = ratesController.state;
       expect(ratesController).toBeDefined();
       expect(fiatCurrency).toBe('usd');
-      expect(Object.keys(rates)).toStrictEqual([Cryptocurrency.Btc]);
-      expect(cryptocurrencies).toStrictEqual([Cryptocurrency.Btc]);
+      expect(Object.keys(rates)).toStrictEqual([
+        Cryptocurrency.Btc,
+        Cryptocurrency.Solana,
+      ]);
+      expect(cryptocurrencies).toStrictEqual([
+        Cryptocurrency.Btc,
+        Cryptocurrency.Solana,
+      ]);
     });
   });
 
@@ -119,11 +125,16 @@ describe('RatesController', () => {
       const publishActionSpy = jest.spyOn(messenger, 'publish');
 
       jest.spyOn(global.Date, 'now').mockImplementation(() => getStubbedDate());
-      const mockRateValue = 57715.42;
+      const mockBtcRateValue = 57715.42;
+      const mockSolRateValue = 200.48;
+
       const fetchExchangeRateStub = jest.fn(() => {
         return Promise.resolve({
           btc: {
-            eur: mockRateValue,
+            eur: mockBtcRateValue,
+          },
+          sol: {
+            eur: mockSolRateValue,
           },
         });
       });
@@ -144,6 +155,10 @@ describe('RatesController', () => {
           conversionDate: 0,
           conversionRate: 0,
         },
+        sol: {
+          conversionDate: 0,
+          conversionRate: 0,
+        },
       });
 
       await ratesController.start();
@@ -163,7 +178,11 @@ describe('RatesController', () => {
       expect(ratesPosUpdate).toStrictEqual({
         btc: {
           conversionDate: MOCK_TIMESTAMP,
-          conversionRate: mockRateValue,
+          conversionRate: mockBtcRateValue,
+        },
+        sol: {
+          conversionDate: MOCK_TIMESTAMP,
+          conversionRate: mockSolRateValue,
         },
       });
 
@@ -326,7 +345,10 @@ describe('RatesController', () => {
 
       const cryptocurrencyListPreUpdate =
         ratesController.getCryptocurrencyList();
-      expect(cryptocurrencyListPreUpdate).toStrictEqual([Cryptocurrency.Btc]);
+      expect(cryptocurrencyListPreUpdate).toStrictEqual([
+        Cryptocurrency.Btc,
+        Cryptocurrency.Solana,
+      ]);
       // Just to make sure we're updating to something else than the default list
       expect(cryptocurrencyListPreUpdate).not.toStrictEqual(
         mockCryptocurrencyList,

packages/assets-controllers/src/RatesController/RatesController.ts

@@ -12,8 +12,15 @@ import type {
 
 export const name = 'RatesController';
 
+/**
+ * Supported cryptocurrencies that can be used as a base currency. The value needs to be compatible
+ * with CryptoCompare's API which is the default source for the rates.
+ *
+ * See: https://min-api.cryptocompare.com/documentation?key=Price&cat=multipleSymbolsPriceEndpoint
+ */
 export enum Cryptocurrency {
   Btc = 'btc',
+  Solana = 'sol',
 }
 
 const DEFAULT_INTERVAL = 180000;
@@ -31,8 +38,12 @@ const defaultState = {
       conversionDate: 0,
       conversionRate: 0,
     },
+    [Cryptocurrency.Solana]: {
+      conversionDate: 0,
+      conversionRate: 0,
+    },
   },
-  cryptocurrencies: [Cryptocurrency.Btc],
+  cryptocurrencies: [Cryptocurrency.Btc, Cryptocurrency.Solana],
 };
 
 export class RatesController extends BaseController<