Add Consumer based variant methods to HttpRequestValues builder

[rstoyanchev]

Consumer based methods for headers, cookies, and attributes provide direct access to the underlying map that in turn provides a richer API as well as visibility into existing values. Similar methods exist in RestClient and WebCilent.

Requested by Spring Security, which has an existing Consumer based API for attributes, in order to build on the HttpRequestValues.Processor hook added recently in #34699.