Support base URL with path

[nlordell]

#132 introduced regressions that caused the order alerter to constantly fail to retrieve orders. This is because the public API URL contains /$network path prefix that wasn't supported by the alerter.

This PR adds support for this. Basically, it just joins instead of replacing the path entirely with set_path.

Test Plan

Run the alerter and see it make successful API requests:

% cargo run -p alerter -- --min-order-age 1 --time-without-trade 1
    Finished dev [unoptimized + debuginfo] target(s) in 0.09s
     Running `target/debug/alerter --min-order-age 1 --time-without-trade 1`
2022-04-07T11:24:09.456Z  INFO alerter: running alerter with Arguments {
    time_without_trade: 1s,
    min_order_age: 1s,
    min_alert_interval: 1799.99997952s,
    errors_in_a_row_before_alert: 5,
    orderbook_api: "https://api.cow.fi/mainnet/",
}
2022-04-07T11:24:09.701Z DEBUG alerter: found 16 open orders
2022-04-07T11:24:09.701Z DEBUG alerter: found no fulfilled orders
2022-04-07T11:24:39.747Z DEBUG alerter: found 14 open orders
2022-04-07T11:24:39.802Z DEBUG alerter: found no fulfilled orders
2022-04-07T11:24:41.905Z DEBUG alerter: 0x url="https://api.0x.org/swap/v1/price?sellToken=0x6b175474e89094c44da98b954eedeac495271d0f&buyToken=0xdac17f958d2ee523a2206206994597c13d831ec7&sellAmount=811063811362965485568" response=Response { sell_amount: 811063811362965485568, buy_amount: 810420605 }
2022-04-07T11:24:43.441Z DEBUG alerter: 0x url="https://api.0x.org/swap/v1/price?sellToken=0x6b175474e89094c44da98b954eedeac495271d0f&buyToken=0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48&sellAmount=475833914861854859264" response=Response { sell_amount: 475833914861854859264, buy_amount: 474553897 }
2022-04-07T11:24:43.952Z DEBUG alerter: 0x url="https://api.0x.org/swap/v1/price?sellToken=0xab846fb6c81370327e784ae7cbb6d6a6af6ff4bf&buyToken=0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2&sellAmount=45974334753652705821" response=Response { sell_amount: 45974334753652705821, buy_amount: 19342714737093254 }
2022-04-07T11:25:13.994Z DEBUG alerter: found 13 open orders
2022-04-07T11:25:14.084Z DEBUG alerter: found no fulfilled orders
2022-04-07T11:25:15.393Z DEBUG alerter: 0x url="https://api.0x.org/swap/v1/price?sellToken=0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48&buyToken=0xdac17f958d2ee523a2206206994597c13d831ec7&sellAmount=220029560" response=Response { sell_amount: 220029560, buy_amount: 218997589 }
...

[nlordell]

Unrelated change, but I needed it when building the alerter crate individually, otherwise:

% cargo build -p alerter
...
   Compiling alerter v0.1.0 (/Users/nlordell/Developer/cowprotocol/services/crates/alerter)
error: The default runtime flavor is `multi_thread`, but the `rt-multi-thread` feature is disabled.
   --> crates/alerter/src/main.rs:277:1
    |
277 | #[tokio::main]
    | ^^^^^^^^^^^^^^
    |
    = note: this error originates in the attribute macro `tokio::main` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `alerter` due to previous error