feat: add horse-provider-nghttp2 provider (HTTP/2 + gRPC, Delphi + FPC)

[freitasjca]

Summary

• Adds HORSE_PROVIDER_NGHTTP2 compiler-define support to Horse.pas — the same pattern used by CrossSocket, mORMot, ICS, epoll and IOCP
• Legacy alias HORSE_NGHTTP2 maps to the canonical define (backwards-compatible)
• Compile-time FATAL guards for all impossible combinations (host-managed + self-hosted, mutual exclusivity between providers)
• uses routing for Console, VCL, and Daemon app shapes on both Delphi and FPC
• THorseProvider alias resolves to THorseProviderNghttp2 when the define is active
• README.md and README.pt-BR.md: table row + installation note (libnghttp2 ≥ 1.59, Boss install, FPC trunk 3.3.1 requirement)

External dependency

horse-provider-nghttp2 + Delphi-nghttp2 — installed via boss install github.com/freitasjca/horse-provider-nghttp2. libnghttp2 is dynamic-loaded at runtime (no link-time dependency). Without {$DEFINE HORSE_PROVIDER_NGHTTP2} the file compiles identically to upstream on both Delphi and FPC.

Test plan

• Compile without any NGHTTP2 define — no change from upstream
• Compile with {$DEFINE HORSE_PROVIDER_NGHTTP2} — routes to Horse.Provider.Nghttp2
• Compile with legacy {$DEFINE HORSE_NGHTTP2} — same result
• Verify FATAL fires for HORSE_PROVIDER_NGHTTP2 + HORSE_HOST_ISAPI (and other invalid combos)
• All existing provider defines unaffected