allow specifying a `fields` Hash to tag collector sources

[majormoses]

Pull Request Checklist

This is similar to #167 but in the context of a source rather than a collector.

General

• [x] Remove any versioning you did yourself if applicable

• [x] Update Changelog following the conventions laid out on Keep A Changelog with all new changes under ## [Unreleased] and using a ### Added, Fixed, Changed, or Breaking Change sub-header.

• [x] Update README with any necessary changes

• [ ] RuboCop passes

• [ ] Foodcritic passes

• [ ] Existing tests pass

Purpose

Allow specifying fields on sources per: https://help.sumologic.com/03Send-Data/Sources/03Use-JSON-to-Configure-Sources

Known Compatibility Issues

[majormoses]

It appears that I need to do some extra work to get my fork updated as it appears to be outdated. It is likely aftermath of renaming master to main.

[majormoses]

see #189 instead