THRIFT-5939: Replace GUID generation with stable UUID algorithm

[Jens-G]

Replace the Windows-only random CoCreateGuid() GUID generation with deterministic UUIDv8 (RFC 9562) derived from SHA-256 over a namespace hierarchy (DNS → thrift.apache.org → program → interface metadata).

Uses SHA-256 implementation by Brad Conte (brad AT bradconte.com), which is explicitly public domain (no copyright claimed). Source: https://github.com/B-Con/crypto-algorithms

• sha256.h: Brad Conte's algorithm verbatim, inlined to a header-only file, macros prefixed THRIFT_SHA256_ to avoid collisions
• Add generate_guid_v8() for services and structs using a canonical string covering: program namespace UUID, entity name, parent service hash (for inheritance), and function/field signatures with Thrift IDL type names
• Add type_name_for_guid(), canonical_service_string(), canonical_struct_string(), program_namespace_uuid() helpers
• Add guid_v4 generator option to fall back to legacy random GUIDs (Windows only, for migration compatibility)
• GUIDs are now emitted on all platforms, not just Windows
• Add UuidV8Test.thrift fixture and CI step that validates determinism, UUIDv8 format, field-order sensitivity, and parent-service hash inclusion
• LICENSE: add attribution section for the third-party public-domain code, following the existing pattern in the file