subscriber: record error sources in fmt layer with JSON format

[stijnherfst]

From #2797 but rebased onto the main branch.

Motivation

Hi, all. As proposed in #2784, this PR records the error sources in fmt layer with JSON format, for both spans and events.

Solution

  Override record_error of fmt::JsonVisitor for span fields.
  Override record_error of serde::SerdeMapVisitor for event fields.
  Use a new field named {field}.sources to record the sources as a list.
  Add a unit test.

Thanks to the original author @BugenZhao!

[SuperFluffy]

I would love if we could get this merged!