Consider supporting labels and selectors for HTTP clients

[philwebb]

Early work on HTTP Service clients included the concept of labels and selectors. These are based on the similar concept in Kubernetes and would allow you to apply labels to specific clients which could later be used to select them.

The concept might still be worth considering at some point in the future, especially if we want to somehow apply specific security properties to specific clients.

The prototype code is at https://github.com/philwebb/spring-boot/commits/interface-client-3, with philwebb@673ee5e, philwebb@e151c56, philwebb@e71f900 and philwebb@5ce83af being the most relevant commits.