Feature request: Parsing MultiPartForm *responses* (currently we can only parse multipart _requests_)

[NHDaly]

(As of HTTP v0.9.17; Julia v1.7.2)

We currently support parsing a multi-part request, but don't support parsing a multi-part response. Here's the multi-part request parsing code:

HTTP.jl/src/parsemultipart.jl

Lines 235 to 246 in 6c0f645

	function parse_multipart_form(req::Request)::Union{Vector{Multipart}, Nothing}
	# parse boundary from Content-Type
	m = match(r"multipart/form-data; boundary=(.*)$", req["Content-Type"])
	m === nothing && return nothing
	boundary_delimiter = m[1]
	# [RFC2046 5.1.1](https://tools.ietf.org/html/rfc2046#section-5.1.1)
	length(boundary_delimiter) > 70 && error("boundary delimiter must not be greater than 70 characters")
	return parse_multipart_body(payload(req), boundary_delimiter)
	end

From what I can tell, this would also work to support responses if we loosened the type restriction on the parameter to HTTP.Message instead of just HTTP.Request. This is what we've done in our own code, and it's been working well:

# Same as HTTP.parse_multipart_form, but this works on both requests and responses as
# opposed to only requests.
# TODO: we should probably PR this to HTTP.jl
function _parse_multipart_form(msg::HTTP.Message)
    # parse boundary from Content-Type
    m = match(r"multipart/form-data; boundary=(.*)$", msg["Content-Type"])
    m === nothing && return nothing

    boundary_delimiter = m[1]

    # [RFC2046 5.1.1](https://tools.ietf.org/html/rfc2046#section-5.1.1)
    length(boundary_delimiter) > 70 && error("boundary delimiter must not be greater than 70 characters")

    return HTTP.MultiPartParsing.parse_multipart_body(HTTP.payload(msg), boundary_delimiter)
end

This is me making good on this "TODO" 😁 Does a PR like that sound okay?

[NHDaly]

(CC: @rgankema, the original author of the above TODO)

[NHDaly]

I've put up a PR for this, here: #817, so there's something concrete to discuss. Thanks! :)