Add support for config file

I have made RPM package for MEV-Boost that is hosted currently at https://copr.fedorainfracloud.org/coprs/jonny/Ethereum/

The source for the SPEC files used to build the RPM can be found here:
https://github.com/hegjon/mev-boost-rpm

The biggest issue is that the configuration and the systemd unit file is in the same file, making it more harder for administrators to change the config. It would be great to be able to ship a systemd unit file that allows updating the package to also reload the service, while keeping the configuration unchanged.

### I have tried to make it better by loading the arguments from EnvironmentFile, but I feel it is only a hack for now:

[mev-boost@.service](https://github.com/hegjon/mev-boost-rpm/blob/f88dcf4da131ed1340b96b89cb1e00012ac843da/sources/mev-boost%40.service#L16)
```
EnvironmentFile=/etc/mev-boost/%i.conf
ExecStart=/usr/bin/mev-boost $MEV_BOOST_ARGS
```

And the [environment file](https://github.com/hegjon/mev-boost-rpm/blob/f88dcf4da131ed1340b96b89cb1e00012ac843da/sources/mainnet.conf#L8) for mainnet:

```
MEV_BOOST_ARGS=" \
-mainnet
-relay-check \
-relay YOUR_RELAY_CHOICE_A \
-relay YOUR_RELAY_CHOICE_B \
-relay YOUR_RELAY_CHOICE_C \
"
```