feat: add connection_cache_size to Config

[fiag]

Add a feature, to set connection_cache_size opt.

[Fishrock123]

At minimum this also need to be implemented for the `async-h1 client, since that will become the default in 3.0.

The docs should also state which other clients this does and doesn't have an effect on (ideally we try to apply it to as many as possible).

[fiag]

Is there any clue to add the connection cache for the async-h1 client?

[Fishrock123]

You'll need to figure out how to implement it for the async-h1 client. You might need an atomic counter of some sort on the client.