lxd/migrate: NewMigrationSink() push mode
lxd/migrate: NewMigrationSink() push mode

Adapt NewMigrationSink() to support push mode. This mainly means that when push
is set to true, NewMigrationSink() will generate a set of websocket secrets
which can be passed back to the client to connect to the dest server.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@canonical.com>

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