Implement `isSelfIssued` for ByronBlock

PR input-output-hk/ouroboros-network#2930 deferred a proper implementation of the new method for Byron, because this new feature is currently only relevant for nodes that are forging blocks on Cardano mainnet, and no such node forges Byron blocks anymore.

But for completeness and eg use in testnets, we should support Byron if it's easy to do so. The essential data is already available in the header, but I'm not sure that the existing `BlockConfig` argument to the `isSelfIssued` method will suffice. The reason I narrowed the method in the PR to `BlockConfig` is because also taking the `LedgerConfig` -- which the simplest hypothetical Byron `BlockSupportsMetrics` instance might currently need -- is that then we'd need an up-to-date `EpochInfo` in order to call this on an HFC block. So be aware of that when determining when to do this.