[pcloud] OpenWriterAt: Default values for multi threaded upload

The associated forum post URL from https://forum.rclone.org

Follow up on #8147

What is your current rclone version (output from rclone version)?

Latest master

What problem are you are trying to solve?

As @ncw described it:

We don't have a way currently for a backend to give a hint as to what chunk size it prefers for OpenWriterAt only for OpenChunkWriter. It would be easy enough to make one though and I think it is a good idea.

How do you think rclone should be changed to solve that?

As far as I see there are two approaches:

1. Add a way for backends to give hints for buffer size etc. in OpenWriterAt. (Similar to ChunkWriterInfo for OpenChunkWriter)
2. Implement OpenChunkWriter in pcloud.

IMHO 1/ is more invasive to the code base but cleaner and consistent with the ChunkWriter. Your thoughts @ncw?

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Start by reading the OpenWriterAt and OpenChunkWriter entry points and the ChunkWriterInfo reference in the backend code, then compare how pcloud handles uploads. Resolve which of the two proposed approaches is accepted; done means pcloud has the desired multi-threaded upload defaults and the relevant backend behavior is covered by tests.