Make it easier to set bufferRequestBody on ClientHttpRequestFactory

[philwebb]

When using RestTemplateBuilder it's currently quite hard to set bufferRequestBody on the ClientHttpRequestFactory

Without this set we get issues such as spring-io/artifactory-resource#50

[kstrijbos]

I would like to have a go at this if possible.

[philwebb]

Thanks @kstrijbos, nobody is currently looking at this so feel free if you have the time.

[philwebb]

Closing in favor of PR #16972