Support for basic authentication as part of the URL

Looks like basic authentication is not supported in dshackle.

Is there any quick way to enable this?

Example:
config:
```yaml
cluster:
upstreams:
- id: "geth-lighthouse"
chain: ethereum
connection:
ethereum:
rpc:
url: "https://username:pass@rpc.lighthouse-geth-001.srv.holesky.ethpandaops.io"
```
dshackle logs:
```sh
2023-28-09 13:58:51.821 | INFO | ConfiguredUpstreams | Using Ethereum upstream, at https://username:pass@rpc.lighthouse-geth-001.srv.holesky.ethpandaops.io
2023-28-09 13:58:52.120 | WARN | EthereumRpcUpstream | Setting up upstream geth-lighthouse with RPC-only access, less effective than WS+RPC
2023-28-09 13:58:52.131 | INFO | EthereumRpcUpstream | Configured for Ethereum
2023-28-09 13:58:52.727 | INFO | entMultistreamHolder | Upstream geth-lighthouse with chain ETHEREUM has been added
2023-28-09 13:58:52.741 | WARN | AbstractHead | Restarting the Head...
2023-28-09 13:58:53.635 | INFO | StarterKt | Started StarterKt in 16.031 seconds (JVM running for 18.811)
2023-28-09 13:59:07.726 | INFO | Multistream | State of ETH: height=?, status=[UNAVAILABLE/1], lag=[0], weak=[geth-lighthouse]
```
Raw curl to the rpc endpoint
```sh
curl --data '{"method":"eth_blockNumber","params":[],"id":1,"jsonrpc":"2.0"}' -H "Content-Type: application/json" -X POST https://username:password@rpc.lighthouse-geth-001.srv.holesky.ethpandaops.io
```
response:
```sh
{"jsonrpc":"2.0","id":1,"result":"0x207"}
```