Include contents of custom assets metadata.yaml in --diagnostics

[Enselic]

Output without custom assets:

Custom assets metadata

Could not read contents of '/Users/martin/.cache/bat/metadata.yaml': No such file or directory (os error 2).

Output with custom assets:

Custom assets metadata

---
bat_version: 0.20.0
creation_time:
  secs_since_epoch: 1646283165
  nanos_since_epoch: 208676000

Remark

When we have access to sharkdp/bugreport#11 we will be able to get even more info for custom assets, but the metadata is a significant giver of valuable information by itself that we can add already now to bat, so let's do that.

This will help slightly with #2085

[sharkdp]

Thanks!