feat: support multiple src= parameters to create multi-source streams

[gudaja]

Summary

Currently, GetOrPatch only accepts a single src query parameter. This PR extends it to accept multiple src parameters, allowing a stream to be created with several producers at once.

Changes

• GetOrPatch now reads all src values from the query string instead of just the first one
• When more than one src is provided, a new multi-source stream is created using all of them as producers
• An optional name parameter can be used to name the stream; if omitted, the first src value is used as the name
• If a stream with that name already exists, it is returned as-is (idempotent)
• Single-source behavior is unchanged

Usage

GET /api/stream?src=rtsp://admin:admin@192.168.1.1/stream0&src=rtsp://admin:admin@192.168.1.1/stream1&name=my-multi-cam

This creates a stream named my-multi-cam backed by two producers: rtsp://cam1 and rtsp://cam2.

Motivation

Some setups require combining multiple video sources into a single logical stream (e.g. redundant cameras, multi-angle feeds). Previously this required manual configuration in go2rtc.yaml. With this change it can be done dynamically via the API.