`rust-server-codegen`: add support for `@httpPayload` in requests (#1060)

This commit adds support for the `httpPayload` Smithy trait when applied
to operation input members (request deserialization).

The code to deserialize HTTP-bound data from HTTP responses in
`ResponseBindingGenerator.kt` has been moved into a common class,
`HttpBindingGenerator.kt`, since it's useful for both clients and
servers in deserializing data from HTTP requests and responses,
respectively.