Feat: Get chunks of response from HTTP client rather than waiting for all data to arive before calling callback

**Notice**
I am working on a project requiring me to work with open AI api and specifically gpt. There is specific type of response from them called streams
https://platform.openai.com/docs/api-reference/chat/create (streaming part).

Point that drogon HTTP client accumulates all of response until all of data was sent without ability to read chunks of data. Would be nice if that would be possible for example by adding additional callback

**Is your feature request related to a problem? Please describe.**
Yes, my problem requires extension of HTTP client to be able to get chunks of response, not only when it all was sent back

**Describe the solution you'd like**
New type of callback which returns chunks of data which arrive at response, rather than accumulate all of it until request is done

**Describe alternatives you've considered**
Alternative solution i have considered - using different http lib, for example lib curl, but would be nice to have such feature inside of drogon.

**Additional context**
Add any other context or screenshots about the feature request here.