feat: migrate wasi_crypto #29

Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Blocked by the pilot migration (see Relationships) — follow the recipe in docs/MIGRATING.md once it lands.

Migrate wasi_crypto to build, test, and ship from this repository.

• Enable wasi_crypto in the standalone CMake build (its existing WASMEDGE_PLUGIN_* option) against the pinned WasmEdge SDK
• Port tests from test/plugins/wasi_crypto and make them pass locally — or add a smoke test if no upstream suite exists
• Add CI following the pilot pattern: .github/workflows/build-wasi_crypto.yml + .github/scripts/wasi_crypto/
• Add the plugin row to README.md
• After the first release containing wasi_crypto: tick it off in the core-removal tracking issue

Migration notes: needs OpenSSL >= 3.0; also uses the imported thirdparty/wasi_crypto spec headers.

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