Implement local socket to communicate with finder extension

[claucambra]

Signed-off-by: Claudio Cambra claudio.cambra@gmail.com

This PR changes how we handle the communication with the Finder extension through our socket. The socket client now lives in the Finder extension.

The biggest benefits of this change are that:

1. We can easily use the macOS' native grand central dispatch mechanisms to make async calls for reading and writing our data without blocking our main app thread, improving the performance of the socketapi-related code
2. We can get rid of the deprecated NSDistantObject usage on socketapisocket_mac
3. We can use a QLocalServer for our socket on the Mac, like we do on other platforms, instead of a custom SocketApiServer
4. We have less Mac-specific code in our application socket code, and Mac code lives with Mac code in the Finder Sync extension
5. The architecture of our Finder extension more closely resembles that of our other shell integration components

[claucambra]

Might also fix #4277

[mgallien]

did not really test right now
sounds nice and good
thanks !

[claucambra]

/rebase

[nextcloud-desktop-bot]

AppImage file: Nextcloud-PR-4316-628ee10008917cc73d9ae1daaacde66285176698-x86_64.AppImage

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[sonarcloud]

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