data_channels 項目に header 項目を追加

[torikizi]

data_channels 項目に header 項目を追加しました。
dc.header は string から json にパースして設定しています。

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This pull request introduces a new Header property to the DataChannel class and updates related files to support this addition. The most important changes include updates to the Sora.Config, DataChannel class, and associated methods in both C# and C++ code, as well as the protocol buffer definition.

Enhancements to DataChannel:

• Sora/Sora.cs: Added Header property to the DataChannel class.
• Sora/Sora.cs: Updated Connect method to set the Header property if it is not null.

Protocol buffer updates:

• proto/sora_conf_internal.proto: Added header field to the DataChannel message.

C++ code updates:

• src/sora.cpp: Updated DoConnect method to parse and set the Header property from JSON.

Documentation updates:

• CHANGES.md: Documented the addition of the Header property to DataChannels.