reverseproxy: make stream copy buffer size configurable

[steadytao]

Implements #7624 by making the copy buffer size for upgraded bidirectional streams configurable.

Summary

This adds a new stream_buffer_size option to reverse_proxy.

It applies to upgraded streaming connections such as websockets where Caddy currently uses io.Copy in both directions. Since io.Copy allocates a 32 KiB buffer per copy operation, long-lived mostly-idle websocket connections can end up holding roughly 64 KiB of buffer memory per connection.

With this change, the buffer size for those streaming copies can be tuned explicitly.

Example:

reverse_proxy localhost:8000 {
    stream_buffer_size 4KB
}

Behavior

• stream_buffer_size controls the buffer used for each direction of an upgraded stream
• if unset or zero, the current default behavior remains unchanged (32 KiB)
• this only affects upgraded bidirectional streams such as websockets
• normal request/response buffering is unchanged

Tests

Added:

• unit coverage for the streaming copier buffer sizing
• Caddyfile adaptation coverage for stream_buffer_size

Implements + should close #7624

Assistance Disclosure

No AI was used.