Add a mode to the receive buffer to handle external buffers

[guhetier]

Description

As part of #4747, the receive buffer needs a mode to operate on buffers provided by the application.
This PR defines a new QUIC_RECV_BUF_MODE_EXTERNAL mode.

• QUIC_RECV_CHUNKs now have a pointer to the actual buffer instead of an end-of-struct array
  • Except for the EXTERNAL mode, the buffer is still part of the same allocation as the chunk
  • For EXTERNAL mode, buffers are provided by the application and chunks point to them
• The EXTERNAL mode writes only once to the provided buffers and discard them when they are drained. Buffers are never re-used.
• Reads and writes can span through multiple chunks. They are never treated as circular buffers.
• The EXTERNAL mode never allocates / re-allocate buffers
  • The virtual buffer size is set to the sum of the provided buffer sizes to avoid receiving more data than we can buffer
  • The virtual buffer size can now decrease when data is drained - but the maximum byte offset that can be received still only increases.

Testing

To be done

Documentation

To be done