feat: add ChannelMerge

[hhu-cc]

add two func ChannelCollector and BufferedChannelCollector for channel helper,
collect messages from multiple input channels into one channel.

[samber]

Hi @hhu-cc and thanks for this first contribution

I would name it ChannelMerge or ChannelAggregation instead of ChannelCollector, because the "collection" could be interpreted as a message source or an observable.

WDYT ? any other naming idea ?

Also, I wonder if it is a good idea to create an unbuffered helper.

[hhu-cc]

Hi @samber
ChannelMerge is a better idea, I have already changed the func name and removed the unbuffered helper.