Add a setting for the max number of parallel uploads

[AinzLimuru]

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Feature request

Is your feature request related to a problem? Please describe.
Supports setting the number of concurrent uploads, which can be implemented on both the client and the server. For the Raspberry Pi, a concurrent number of more than 5 may be unbearable, but for a powerful server, a concurrent number of 50 may not be able to fully utilize the performance.
My situation belongs to the latter. I need a larger concurrent number, especially my local file size is small but the total amount is large. The current concurrent number is a disaster for my photo synchronization. I have tens of thousands of pictures, but the efficiency is very low due to remote synchronization delays and limited concurrency.

Describe the solution you'd like
Supports setting the number of concurrent uploads, which can be implemented on both the client and the server.

Describe alternatives you've considered
None

Additional context
None