Add an argument to specify client type / chain provider

[sergerad]

Currently the CLI works as follows:

$ blockrs -h                                                                                                                       
TUI for tailing chain data

Usage: blockrs [OPTIONS] <RPC_URL>

Arguments:
  <RPC_URL>  ...

In anticipation of adding support for other chain providers beyond Eth RPC, we should add an argument before the <RPC_URL> to specify which provider to use. This should map to the provider implementation in the providers dir.

The new usage should look like this:

Usage: blockrs [OPTIONS] <PROVIDER> <RPC_URL>

The README.md should be updated to the effect of:

blockrs eth https://rpc.flashbots.net