Support configuring trino-lb endpoint instead of address

Came up in https://github.com/stackabletech/trino-lb/pull/100#discussion_r2753723253

Basically, change

trinoLb:
  externalAddress: https://127.0.0.1:8443

to

trinoLb:
  externalEndpoint: https://127.0.0.1:8443/my/subpath

However, this is a breaking change and needs some rework of trino-lb.
Think of e.g.

fn change_next_uri_to_trino_lb(next_uri: &Url, trino_lb_addr: Url) -> Url {
    let mut result = trino_lb_addr;
    result.set_path(next_uri.path());
    result
}

This obviously needs to be changed to handle subpaths in the trino-lb endpoint

Start by tracing the trinoLb externalAddress configuration and the change_next_uri_to_trino_lb function shown in the issue. Review how the endpoint is parsed and applied, then update the surrounding behavior so a configured externalEndpoint, including its subpath, is preserved when constructing the next URI; verify the resulting routing behavior with the relevant existing checks.