Add CHANGELOG support for multiple reference authors (#2012)

## Motivation and Context

Currently only a single author can be provided to a changelog entry.
This does not support the cases where a single PR was coauthored by
multiple authors and/or multiple PRs were authored by different
individuals.

## Description

- Accept a list of strings, in addition to a single string in the
`author` field. Each author in the list is added as a contributor to
every PR provided in references.
- Accept references of the form `{ "id": <ID>, "authors": <Author> }`,
in addition to `ID`. Each author given in `authors` is added as a
contributor to that PR. Authors common to all references will be
serialized in the same way as using the top-level `author` field.

## Notes

- This should not be a breaking change. Existing `CHANGELOG.next.toml`
should be deserialized and then serialized identically.

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Co-authored-by: Harry Barber <hlbarber@amazon.co.uk>
Co-authored-by: John DiSanti <jdisanti@amazon.com>