Add support for cargo features to playground server

[mcintyre94]

In Seahorse when using Pyth, it is an optional dependency that is enabled using anchor build -- --features pyth-sdk-solana

This enables code in the generated src/lib.rs:

    #[cfg(feature = "pyth-sdk-solana")]
    pub use pyth_sdk_solana::{load_price_feed_from_account_info, PriceFeed};

I think for Playground to compile this code we'd need to have it pass the feature flag.

I think ideally we should be able to pass features: ["pyth-sdk-solana"] to the /build server, alongside rust files + uuid, and then have the build server include this feature flag when building

One thing I'm not sure about is how dependencies work in the build server, because we don't pass cargo.toml in. I guess the build server would need to detect the pyth dependency only if that feature flag is passed in?

[mcintyre94]

I'll close this, we don't need this for seahorse features any more and doesn't seem to have been any other interest in the idea. Probably not worth doing!