Implement Future for Builder

[soedirgo]

Currently, users have to call execute at the end of a request:

let resp = client
    .from("table")
    .select("*")
    .execute()
    .await?;

To omit this, we need to implement the Future trait, which opens a can of worms that I really don't want to deal with right now. For now, the plan is to cross fingers and wait for this to be implemented directly into reqwest's RequestBuilder (see issue 908 in reqwest).