Return readonly: false for local keystores (#3490)

## Issue Addressed

NA

## Proposed Changes

Indicate that local keystores are `readonly: Some(false)` rather than `None` via the `/eth/v1/keystores` method on the VC API.

I'll mark this as backwards-incompat so we remember to mention it in the release notes. There aren't any type-level incompatibilities here, just a change in how Lighthouse responds to responses.

## Additional Info

- Blocked on #3464

validator_client/src/http_api/keystores.rs

@@ -40,7 +40,7 @@ pub fn list<T: SlotClock + 'static, E: EthSpec>(
                     SigningMethod::LocalKeystore {
                         ref voting_keystore,
                         ..
-                    } => (voting_keystore.path(), None),
+                    } => (voting_keystore.path(), Some(false)),
                     SigningMethod::Web3Signer { .. } => (None, Some(true)),
                 });