[kotlin][client] support text/plain in okhttp

[Djaler]

Fixes #15973

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[wing328]

thanks for the PR, which looks good to me

cc @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10)

[ranger-ross]

Thank you for implementing this. I ran into this last week and I was quiet confused.
I'll be looking forward to this in 7.11.0 ❤️

[4brunu]

@Djaler did you deleted OctetMediaType by accident or on propose?

[Djaler]

On purpose. See message for this commit c5836ff