http: support Content-Encoding=deflate

http: support Content-Encoding=deflate

Add support for compressing the body of responses with
`Content-Encoding: deflate` following [RFC9110]. Note that in this
context `deflate` is actually a "zlib" data format as defined in
[RFC1950].

To preserve the current behavior we prefer `Content-Encoding: gzip`
whenever `gzip` is listed as one of the encodings in the
`Accept-Encoding` header and the data should be compressed.

[RFC9110] https://www.rfc-editor.org/rfc/rfc9110#name-deflate-coding
[RFC1950] https://www.rfc-editor.org/rfc/rfc1950

Suggested-by: Lukas Wagner <l.wagner@proxmox.com>
Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Tested-by: Folke Gleumes <f.gleumes@proxmox.com>