Feature request: Calculate ETA based on size of remaining files + speed for file size

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

N/A (I'm just creating this issue to get comments/suggestions before implementing it, as CONTRIBUTING.md said I should)

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

rclone v1.60.0
- os/version: darwin 12.5.1 (64 bit)
- os/kernel: 21.6.0 (arm64)
- os/type: darwin
- os/arch: arm64
- go/version: go1.19.2
- go/linking: dynamic
- go/tags: none

What problem are you are trying to solve?

When an operation (copying an either home dir) has a mix of a lot of small files as well as big files, the ETA is inaccurate, jumping from several hours to 14 weeks.

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

Having file size "brackets" (<1KB, <1MB, <100MB, <1GB, <10GB) with the current rolling average applied to each bracket (files are placed in the brackets as they're transferred, and after there's enough info, the ETA is calculated for all remaining files. This should be possible as rclone already scans for all files before starting any transferring so it should have the file sizes of each file. To keep memory usage low, just a count of files in each bracket would be kept, not assigning a bracket on a file-by-file basis.)

I'd like to try and implement this if it seems to be a good feature for rclone.

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Start by locating rclone's transfer progress and ETA calculation in the Go code, then trace how the initial file scan and rolling transfer speed are represented. Compare the current estimate with mixed small and large files; done means the ETA uses remaining file-size groups and tests cover that scenario.