RFR: 7903271: Add sample of libzstd

[Per Minborg]

This PR adds a sample using `libzstd`, a high-performance compression library.

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Commit messages:
 - Add sample of libzstd

Changes: https://git.openjdk.org/jextract/pull/66/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=66&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903271
  Stats: 158 lines in 8 files changed: 158 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/66.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/66/head:pull/66

PR: https://git.openjdk.org/jextract/pull/66