Rework relay CLI a bit

[tomusdrw]

This PR extracts some common parts of the CLI commands to avoid duplication and also changes the way CLI is structured a bit.

Currently the CLI is flat:

$ ./target/release/substrate-relay --help
substrate-relay 0.1.0
Substrate-to-Substrate relay

USAGE:
    substrate-relay <SUBCOMMAND>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

SUBCOMMANDS:
    help                                          Prints this message or the help of the given subcommand(s)
    initialize-millau-headers-bridge-in-rialto    Initialize Millau headers bridge in Rialto
    initialize-rialto-headers-bridge-in-millau    Initialize Rialto headers bridge in Millau
    millau-headers-to-rialto                      Relay Millau headers to Rialto
    millau-messages-to-rialto                     Serve given lane of Millau -> Rialto messages
    rialto-headers-to-millau                      Relay Rialto headers to Millau
    rialto-messages-to-millau                     Serve given lane of Rialto -> Millau messages
    submit-millau-to-rialto-message               Submit message to given Millau -> Rialto lane
    submit-rialto-to-millau-message               Submit message to given Rialto -> Millau lane

IMHO it becomes already a bit difficult to parse long strings, and since I'm planning to add even more commands (see #607, #608, #609) I've changed it to have a bit more structure:

$ ./target/release/substrate-relay --help
substrate-relay 0.1.0
Substrate-to-Substrate relay

USAGE:
    substrate-relay <SUBCOMMAND>

FLAGS:
    -h, --help       
            Prints help information

    -V, --version    
            Prints version information


SUBCOMMANDS:
    help              Prints this message or the help of the given subcommand(s)
    init-bridge       Initialize on-chain bridge pallet with current header data
    relay-headers     Start headers relay between two chains
    relay-messages    Start messages relay between two chains
    send-message      Send custom message over the bridge

Initially I tried to split the CLI into network related commands (like $ relay millau to-rialto headers), but this quickly get's out of hand and is not user friendly.
Instead in the top level you will get different functions of the relayer binary first and then you see networks.

• I wanted to avoid doing too much changes, so the commands are still in a single module, but I think the next step might be extracting commands/networks/bridges into separate modules.

• Another thing worth trying in the future is avoiding duplication via macro, but while it will remove duplication I think it won't make the code more readable/maintainable, hence after a couple of failed attempts I didn't follow that direction further.

[adoerr]

LGTM

[adoerr]

ses uses