Add IOBuf<->std::iostream adapters for zero-copy I/O

[chenBright]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

IOBuf integrates with protobuf via IOBufAsZeroCopy{Input,Output}Stream, but
there is no direct bridge to the std::iostream hierarchy. As a result, any
library that consumes/produces std::istream& / std::ostream& — most notably
nlohmann::json — cannot read from or write to an IOBuf without an
intermediate std::string:

auto j = nlohmann::json::parse(req->request_attachment().to_string());

This is an obstacle to adopting nlohmann::json in bRPC.

What is changed and the side effects?

Changed:

Introduce four classes in src/butil/iobuf.{h,cpp} that bridge IOBuf and
the standard iostream hierarchy:

• IOBufAsInputStreamBuf — read-only std::streambuf over an IOBuf,
  walking the backing blocks via the public backing_block() API. Overrides
  underflow, xsgetn(bulk memcpy across block boundaries to avoid the
  default per-byte sbumpc loop), and showmanyc (saturating sum of
  remaining block sizes).
• IOBufInputStream — std::istream view that wires the streambuf above.
• IOBufAsOutputStreamBuf — append-only std::streambuf that delegates to
  the existing IOBufAsZeroCopyOutputStream for block allocation and the
  three-branch BackUp (including the shared-block degradation that protects
  IOBufs sharing blocks from being corrupted by an in-flight write). Overrides
  overflow, xsputn (block-sized memcpy instead of per-byte sputc),
  and sync (forwards to shrink()).
• IOBufOutputStream — std::ostream view that wires the streambuf above;
  accepts an optional block_size to allocate dedicated blocks instead of
  drawing from the per-thread TLS pool.

Usage with nlohmann::json:

  // Parse: zero string copy
  butil::IOBufInputStream in(request_body);
  auto j = nlohmann::json::parse(in);

  // Serialize: bytes flow straight into IOBuf blocks
  butil::IOBufOutputStream os(response_body);
  os << j;

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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