feat: Init Staking API GraphQL Plugin, discontinue Binance Spot

[rossbulat]

Introduces Staking API Plugin, a GraphQL service that is hosted at https://api.staking.polkadot.cloud.

• [x] Binance Spot API has been discontinued in favour of fetching the DOT & KSM price from Staking API.
• [x] Added plugin-staking-api to handle the Staking API GraphQL service.
• [x] Network bar and balance module now utilise the Staking API plugin.
• [x] Removes redundant VITE_DISABLE_FIAT environment variable.
• [x] Removes redundant usePrices and useUnitPrice hook.