Support arbitrary sidecar files

[qlyoung]

Currently support exists for handling .xmp sidecar files. This change expands that support to any arbitrary sidecar file. It also expands the set of sidecars which are handled by default. The new default set, and the reason this set was chosen, is:

• .xmp - general metadata format for all image types; written by Lightroom, Digikam, Darktable, etc
• .json - generated by google takeout when exporting from g photos
  • .yml/.yaml - used by Photoprism

Users can override this default set by passing the new --sidecars option. For example, to handle .xyz, users should pass --sidecars="xmp,json,yml,xyz"; and to disable sidecar handling altogether, pass --sidecars=''

As with my other patch, I'm submitting this without tests so we can reach consensus about the feature.

Example

Before the change, .json sidecars are placed in unknown rather than being moved with their images:

[2023-08-31 01:20:39] - [INFO] - /mnt/in/2018/07/20180713_130947_D3B693D9.jpg => /mnt/out/2018/07/13/20180713-130947252564.jpg
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_130947_D3B693D9.json => /mnt/out/unknown/20180713_130947_D3B693D9.json
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_162837_1D565480.jpg => /mnt/out/2018/07/13/20180713-162837.jpg
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_162837_1D565480.json => /mnt/out/unknown/20180713_162837_1D565480.json
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_164015_86C6782E.jpg => /mnt/out/2018/07/13/20180713-164015.jpg
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_164015_86C6782E.json => /mnt/out/unknown/20180713_164015_86C6782E.json
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_171837_4104A370.jpg => /mnt/out/2018/07/13/20180713-171837205064.jpg
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_171837_4104A370.json => /mnt/out/unknown/20180713_171837_4104A370.json
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_171912_7AFD55D7.jpg => /mnt/out/2018/07/13/20180713-171912555982.jpg
[2023-08-31 01:20:40] - [INFO] - /mnt/in/2018/07/20180713_171912_7AFD55D7.json => /mnt/out/unknown/20180713_171912_7AFD55D7.json

After, they're handled correctly:

[2023-08-31 01:14:35] - [INFO] - /mnt/in/2018/07/20180713_130947_D3B693D9.jpg => /mnt/out/2018/07/13/20180713-130947252564.jpg
[2023-08-31 01:14:35] - [INFO] - /mnt/in/2018/07/20180713_130947_D3B693D9.json => /mnt/out/2018/07/13/20180713-130947252564.json
[2023-08-31 01:14:35] - [INFO] - /mnt/in/2018/07/20180713_162837_1D565480.jpg => /mnt/out/2018/07/13/20180713-162837.jpg
[2023-08-31 01:14:35] - [INFO] - /mnt/in/2018/07/20180713_162837_1D565480.json => /mnt/out/2018/07/13/20180713-162837.json
[2023-08-31 01:14:35] - [INFO] - /mnt/in/2018/07/20180713_164015_86C6782E.jpg => /mnt/out/2018/07/13/20180713-164015.jpg
[2023-08-31 01:14:35] - [INFO] - /mnt/in/2018/07/20180713_164015_86C6782E.json => /mnt/out/2018/07/13/20180713-164015.json
[2023-08-31 01:14:36] - [INFO] - /mnt/in/2018/07/20180713_171837_4104A370.jpg => /mnt/out/2018/07/13/20180713-171837205064.jpg
[2023-08-31 01:14:36] - [INFO] - /mnt/in/2018/07/20180713_171837_4104A370.json => /mnt/out/2018/07/13/20180713-171837205064.json
[2023-08-31 01:14:36] - [INFO] - /mnt/in/2018/07/20180713_171912_7AFD55D7.jpg => /mnt/out/2018/07/13/20180713-171912555982.jpg