Make Request mutable for HttpClient

The main reason we have `Clone` constraints on request-related types is because the `HttpClient` trait requires an immutable `Request`, but `reqwest::Client` wants a mutable request. The original intent - as found in the `legacy` branch - was that a client - the transport, really - shouldn't mutate the request. In concept that makes sense: only `Policy` implementations were intended to do that. However, this prevents us from effectively streaming request bodies without jumping through hooks like in #4491.

Should we ever cut a 2.0 release with breaking changes, we should reconsider making the `Request` mutable, as would be necessary to stream the request body. This should allow us to remove the `Clone` constraint and clean up a lot of code.

Start by reviewing the legacy branch behavior and issue #4491, which are the references provided for the current request-mutation and streaming constraints. The work would need a decided 2.0 API direction, including whether Request becomes mutable and how the Clone constraints are removed; this issue does not identify files or tests.

Written by the indexing model from the issue text.