Implement feerecipient API for keymanager (#3213)

## Issue Addressed

* #3173 

## Proposed Changes

Moved all `fee_recipient_file` related logic inside the `ValidatorStore` as it makes more sense to have this all together there. I tested this with the validators I have on `mainnet-shadow-fork-5` and everything appeared to work well. Only technicality is that I can't get the method to return `401` when the authorization header is not specified (it returns `400` instead). Fixing this is probably quite difficult given that none of `warp`'s rejections have code `401`.. I don't really think this matters too much though as long as it fails.

validator_client/src/cli.rs

@@ -136,14 +136,6 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .value_name("FEE-RECIPIENT")
                 .takes_value(true)
         )
-        .arg(
-            Arg::with_name("suggested-fee-recipient-file")
-                .long("suggested-fee-recipient-file")
-                .help("The fallback address provided to the BN if nothing suitable is found \
-                           in the validator definitions.")
-                .value_name("FEE-RECIPIENT-FILE")
-                .takes_value(true)
-        )
         /* REST API related arguments */
         .arg(
             Arg::with_name("http")