Is there a way to install a plugin on vCO automatically?
There is a REST API to import a plug-in file (.dar or .vmoapp).
(see REST API docs for details)
However, after the plug-in is imported, you still need to restart vCO server (and there is no API to restart the server yet).
I gone through docs. It says that the plugin binary contents must be available as multipart content.
And it is binary content.
Do you have any idea how I can convert .vmoapp or .dar to binary?
.vmoapp and .dar are already binaries.
'Multipart content' is a type of HTTP request that HTTP clients/libraries construct when needed to send files/data to the server.
How to prepare multipart request is language/library dependent so there is no common recipe for it.
You can do like it is done in this post for uploading a workflow : https://communities.vmware.com/message/2331834
If you don't have a library for preparing the multipart content you can do it by yourself, there is an example in the mentionned thread.