build: opt-in C++20 (MINIO_CPP_STD=20) to unlock span/format/coroutines

[harshavardhana]

Problem

Default is `MINIO_CPP_STD=17` (`CMakeLists.txt:54`). That blocks several modernization wins:

• `std::spanstd::byte` for buffer params (replaces the `char* buf; size_t size;` pairs in `args.h:162-183`)
• `std::format` / `std::to_chars` ergonomics for hex/date formatting (see perf: replace O(n²) hex-encoding loops with std::to_chars #219)
• Coroutine-ready awaitables in the async surface (see api: async Get/Put surface (std::future or coroutine-ready awaitables) #217 Phase 2)
• `std::ranges` for any iterator-loop pipelines (low priority)

Suggested approach

Don't force the upgrade — make it opt-in:

```cmake
set(MINIO_CPP_STD "17" CACHE STRING "C++ standard (17 or 20)")
```

Then guard the C++20-only paths with `#if __cplusplus >= 202002L` (the codebase already has one such guard at `include/miniocpp/utils.h:173` for `operator<=>`).

Public API stays C++17-compatible by default. C++20 builds get the extras.

Impact

Enabling change. No perf/correctness improvement by itself — but unblocks #217 Phase 2, span-based buffer APIs, and potentially #222 (`std::expected` is C++23 but `tl::expected` is C++17 — so #222 doesn't strictly require this).

Roadmap

Tier 3 item from the C++ modernization audit.