Fix "direct buffer access" issue for Plugin SDK

[marrodion]

Description

Fixes ModelServerRequest.getInputStream() issue when the underlying buffer is a Netty direct buffer.

Fixes #2498

Type of change

[x] Bug fix (non-breaking change which fixes an issue)

Feature/Issue validation/testing

Added a simple test for direct buffer write/read using SDK abstractions.
Not sure if a new test suite is justified, would be happy to move elsewhere.

Checklist:

• Did you have fun?
• Have you added tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

[codecov]

Codecov Report

Merging #2511 (20c6882) into master (53ff6a5) will not change coverage.
The diff coverage is n/a.

❗ Current head 20c6882 differs from pull request most recent head 4534636. Consider uploading reports for the commit 4534636 to get more accurate results

@@           Coverage Diff           @@
##           master    #2511   +/-   ##
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  Coverage   72.66%   72.66%           
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  Files          78       78           
  Lines        3669     3669           
  Branches       58       58           
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  Hits         2666     2666           
  Misses        999      999           
  Partials        4        4           

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