Allow reuse of library-specific configuration code in `ClientHttpRequestFactoryFactory` and `ClientHttpConnectorFactory`

[mp911de]

To be able to reuse existing configuration code in the sense of applying Spring's defaults with the ability to add application-specific configuration it would be good to be able to use e.g. ClientHttpRequestFactoryFactory.HttpComponents to obtain a builder for the HTTP client.

Applications could leverage that code without the need to duplicate ClientHttpRequestFactoryFactory and friends and configure the HTTP client with specific additional options while retaining Spring's defaults.