add perp/spot market update notifcations

[wphan]

More reactive use cases may benefit from being notified whenever a market account is updated. This PR adds DriftClient::perp_market_watch and DriftClient::spot_market_watch which returns a tokio::sync:watch that notifies the market_index of the updated account

[jordy25519]

Suggested change

	pub async fn subscribe_markets(
	&self,
	markets: &[MarketId],
	on_account: Option<Arc<Box<OnAccountFn>>>,
	) -> SdkResult<()> {
	self.backend.subscribe_markets(markets, on_account).await
	pub async fn subscribe_markets_with_callback(
	&self,
	markets: &[MarketId],
	on_account: Option<Arc<Box<OnAccountFn>>>,
	) -> SdkResult<()> {
	self.backend.subscribe_markets(markets, on_account).await

[jordy25519]

would be good to keep the existing api unchanged, use new method only when user wants the callback