feat(Livestream): Add samples and tests

[irataxy]

b:241866780

Part 2 of 3: Channels. See also https://cloud.google.com/livestream/docs/how-to/create-channels

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 7 region tags.

• media/livestream/api/LiveStream.Samples/CreateChannelAsync.cs:17, tag livestream_create_channel
• media/livestream/api/LiveStream.Samples/DeleteChannelAsync.cs:17, tag livestream_delete_channel
• media/livestream/api/LiveStream.Samples/GetChannel.cs:17, tag livestream_get_channel
• media/livestream/api/LiveStream.Samples/ListChannels.cs:17, tag livestream_list_channels
• media/livestream/api/LiveStream.Samples/StartChannelAsync.cs:17, tag livestream_start_channel
• media/livestream/api/LiveStream.Samples/StopChannelAsync.cs:17, tag livestream_stop_channel
• media/livestream/api/LiveStream.Samples/UpdateChannelAsync.cs:17, tag livestream_update_channel

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