Prep subxt-historic 0.0.7 release: expose ClientAtBlock bits

[jsdw]

Expose OfflineClientAtBlock, OfflineClientAtBlockT, OnlinelientAtBlock, OnlineClientAtBlockT.

This is so that you can pass the ClientAtBlock into functions like so:

use subxt_historic::config::Config;
use subxt_historic::client::{ ClientAtBlock, OnlineClientAtBlock, OnlineClientAtBlockT };

fn accepts_client_at_block_concrete<T: Config>(client: &ClientAtBlock<OnlineClientAtBlock<'_, T>, T>) {
    // ...
}
fn accepts_client_at_block_generic<'conf, T: Config + 'conf, C: OnlineClientAtBlockT<'conf, T>>(client: &ClientAtBlock<C, T>) {
    // ...
}

closes #2137

[TarikGul]

Amazing, thanks :)