[in_http] support additional content_types

**Is your feature request related to a problem? Please describe.**
Not related to a problem

**Describe the solution you'd like**
Update the in_http plugin to support the same content_types as fluentd:
application/x-www-form-urlencoded
multipart/form-data
application/msgpack
application/x-ndjson
application/json - this is the only one seemingly currently supported by the plugin

**Describe alternatives you've considered**
N/A - use of only application/json limits the usefulness of this input plugin

**Additional context**
ref: https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/in_http.rb