[kotlin][client] fix encoding of individual parts of a multipart request

Created by: koscejev

Fixes #11910 (closed)

This propagates param.contentType field from individual API endpoint definitions to the actual ApiClient and uses it to create RequestBody for each individual part with correct Content-Type header as well as correct serialization of the individual part's body. If not specified, the default Content-Type is "application/octet-stream" for files and "application/json" for models - see https://swagger.io/docs/specification/describing-request-body/multipart-requests/. The actual serialization of individual part's body is now reusing code for serialization of the non-multipart body, so it now properly handles many cases.

^ This part was also verified on our own internal project using ok-http4 library only.

Potential questions/risks:

• I'm not 100% sure about how this works for part bodies that are neither files, nor models. But since it reuses code that is handling non-multipart body, so it is consistent with that at least.
• Some samples still contain old code, even though other samples were updated by the specified script? How does this work?
• Should this go to master instead? I will rebase as necessary.

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