This commit makes the `uri` step of the WebClient optional, so that
users who have specified a base URL during WebClient config do not need
to provide an empty one (i.e. `url("")`).
The basic idea of this fix is that the HTTP method methods in WebClient
(`get`, `post`, etc.) should be able to "bypass" the uri stage, and skip
straight to defining headers, or even doing an exchange or retrieve
(i.e. call methods on `RequestHeaderSpec` or `RequestBodySpec`).
I have accomplished this by adding two new composed interfaces:
`RequestHeadersUriSpec` and `RequestBodyUriSpec`.
`RequestHeadersUriSpec` extends from the existing `UriSpec` and
`RequestHeaderSpec`, while `RequestBodyUriSpec` extends from `UriSpec`
and `RequestBodySpec`. These types are returned from the HTTP methods
(`get`, `post` etc). The `uri` methods on these types return a plain
`RequestHeaderSpec` and `RequestBodySpec` (i.e. types without the `uri`
methods), so that you can call `uri` once only.
Issue: SPR-15695
3232cb6260