Set content-type and content-length header for Http Subscription request

*Title*: Set content-type and content-length header for Http Subscription request

*Description*:
Since in here: https://github.com/envoyproxy/envoy/blob/3e15c94901f7ce5ebca671e451efceec8053da86/source/common/config/http_subscription_impl.h#L71 the body is known as JSON and the length is also known, I think it is better to have the content-type and content-length set.
