redactor add: Add optional filtering like env vars

[DrJosh9000]

Description

Adds an --apply-vars-filter flag to the redactor add subcommand, that enables the same filtering as normally happens for filtering env vars for secrets.

Context

Relevant to #3588

Changes

• Adds --apply-vars-filter as described above, and the common --redact-vars flag.
• Updates the tests

Two small cleanups:

• JSON decoding should be able to work with a pointer to a map - the pointer to a pointer to a map should be unnecessary!
• Reordered the unknown format flag so it reads unknown format "foo" rather than foo: unknown format

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

I did it my way