Remove or split MagicOnion.Shared

[mayuki]

Relates to #647

MagicOnion.Shared assembly/package exists independently, but not for Unity.
This causes problems when the generated client is compiled as a DLL and brought into Unity.

We have two options:

• Make all part of Server or Client by source sharing
• Separate MagicOnion.Shared from MagicOnion.Client in the Unity package.

[github-actions]

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[mayuki]

Fixed #707