filter: add ClearFields

[arl]

❓ What

Add ClearFields filter

This filter is required for an AdRoll internal tool. Since it's very generic, I'm adding it to the open-source repository.
It's a simple filter which you can configure giving it a set of record field names, each of those fields are going to be cleared when passing through this filter.

🔨 How to test

1. List all steps necessary;
2. To test this pull request.

✅ Checklists

This section contains a list of checklists for common uses, please delete the checklists that are useless for your current use case (or add another checklist if your use case isn't covered yet).

• Is there unit/integration test coverage for all new and/or changed functionality added in this PR?
• Have the changes in this PR been functionally tested?
• Has make gofmt-write been run on the code?
• Has make govet been run on the code? Has the code been fixed accordingly to the output?
• Have the changes been added to the CHANGELOG.md file?
• Have the steps in CONTRIBUTING.md been followed to update a Go module?

[arl]

Merging this myself. This is just a copy-paste of a filter we use in the compare-tool, an AdRoll internal tool.