feat: Allow providing custom connector to balanced channels by exposing Channel::balance

[Suficio]

Motivation

I have a requirement to balance client gRPC requests via different connectors.

There is currently no API to construct a balanced Channel using a custom connector (it is possible to create a Channel with a custom connector using Endpoint::connect_with_connector_lazy).

Solution

There is currently an internal constructor Channel::balance with the following signature:

pub(crate) fn balance<D>(discover: D, ..) -> Self
where
    D: Discover<Service = Connection>,
    ...

Since Connection is a private type this PR modifies the signature to:

pub fn balance<D, C>(discover: D, ..) -> Self
where
    D: Discover<Service = (C, Endpoint)>,
    C: Service<Uri>,
    ...

This signature supports all internal uses of Channel::balance while allowing maximal flexibility for advanced use cases.