Prior to this commit, HTTP client builders auto-configured by Spring
Boot would be eagerly instantiating resources, even if those were not
used by the application.

This commit makes the `RestTemplateBuilder` bean as Lazy.
`WebClient.Builder` was already a prototype bean, but some of its
dependencies could consume resources, like the `HttpClientConnector` and
the related infrastructure. This commit makes those pieces lazy.

Note that since those components are meant to help instantiate actual
HTTP clients for application components, making them lazy won't make any
difference at runtime since they'll be used during context refresh, or
they won't be used at all.

Closes gh-19549