Consider supporting labels and selectors for HTTP clients

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 https://github.com/philwebb/spring-boot/commit/673ee5e74185ff87ead1b8ca4cd61e2b0efc1fca, https://github.com/philwebb/spring-boot/commit/e151c5636c54bf74cdae8f1d5ef5b457e52d7579, https://github.com/philwebb/spring-boot/commit/e71f9001c687e5ae1c8f827dbdf3479a9e7d8820 and https://github.com/philwebb/spring-boot/commit/5ce83af01eb708d0d382b16d4b622103a6a2dd91 being the most relevant commits.

Start by reviewing the prototype branch and commits 673ee5e, e151c563, e71f900, and 5ce83af to understand the earlier HTTP Service client design. Identify the current HTTP client entry points and determine how labels, selectors, and possible security-property application should fit. The work is done only when the intended behavior and scope are agreed and covered by appropriate tests.