interface: Add get_config_data helper to break circular deps (#60)

interface: Add `get_config_data` helper

#### Problem

The auto-generated clients aren't ready for SDK v3, and Agave is
currently using some auto-generated clients functions for the CLI. The
client libraries currently create a circular dependency with Agave which
we need to break.

#### Summary of changes

Agave can be transitioned to `solana-config-interface` for everything
except the `get_config_data` helper, so add that missing helper.

Author: joncinque

interface/src/state.rs

@@ -14,3 +14,11 @@ pub struct ConfigKeys {
     #[cfg_attr(feature = "serde", serde(with = "short_vec"))]
     pub keys: Vec<(Pubkey, bool)>,
 }
+
+/// Utility for extracting the `ConfigKeys` data from the account data.
+#[cfg(feature = "bincode")]
+pub fn get_config_data(bytes: &[u8]) -> Result<&[u8], bincode::Error> {
+    bincode::deserialize::<ConfigKeys>(bytes)
+        .and_then(|keys| bincode::serialized_size(&keys))
+        .map(|offset| &bytes[offset as usize..])
+}