This commit introduces client-side request attributes, similar to those
found on the server-side. The attributes can be used, for instance, for
passing on request-specific information to a globally registered
ExchangeFilterFunction.
The client request builder, as well as WebClient.RequestHeadersSpec and
WebTestClient.RequestHeaderSpec, add methods for adding a single
attribute, as well as manipulating the entire attributes map.
The client request itself adds a accessor for the (immutable) attributes
map.
This commit also introduces a new variant of the basic authentication
filter in ExchangeFilterFunctions. This variant takes the username and
password from well-known attributes.
Issue: SPR-15691
74b4c02881